• P2P: Peer to peer networks are commonly used for downloading, often illegally, files such as music, movies and games. Since each machine in a P2P network carries part of the overall network, you need to be careful since such networks can be attacked and files can contain viruses or other malware. The network software itself can also become infected.
• Page File: Also known as a swap file and virtual memory. In Windows versions derived from Windows NT, such as XP, the actual file name is pagefile.sys. Rather than being actually virtual memory, this is a paging file used when memory needs exceed the RAM on the machine and data must be swapped out to physical storage on the disk drive. The size of the paging file can be modified to allow large memory intensive functions to run which could otherwise fail. It does, however, use up disk space.
• RAM: Random Access Memory used as the main memory in personal computers. Random access refers to the fact that any piece of data can be gotten in a constant amount of time. Unlike other storage mechanisms like the disk drive where access time is primarily a function of where the piece of data is located and how long it takes to move the medium and/or the read/write head to the location. PC RAM is a form of volatile memory where any information stored is lost if the power is turned off. Since RAM is used by system processes, applications and data, usually the more RAM one has the faster the computer can run because less time is lost moving data in and out of slower disk storage (the page file or virtual memory).
• Registry: The Windows registry was originally developed to provide a central location for program initialization files which were getting a little hard to keep track of. The registry is a directory which stores a large amount of information about the system, the hardware, applications, users, preferences, and so on. The registry has quite a few good points. Here though we are concerned with the main bad points. Damage to it can make your machine unbootable and even unfixable which means a full reinstall of Windows. Applications which don't uninstall properly, or don't have an uninstaller can leave various kinds of junk in the registry which can affect performance or system stability. Editing the registry can be a problem too which is why software applicaitons for registry cleanup are popular. Care always need to be taken when messing around with a Windows registry.
• Restore Point: If you are running a WIndows operating system you can create a file which will allow you to restore your system to a previous configuration. Usually you would do this before making changes to the Registry. If you save a lot of restore points you are probably wasting disk space, so you can delete older restore points to free up space.
• Rootkit: One or more programs which when installed will take control of the computer system without permission. The word is derived from UNIX systems where the highest privilege level is know as "root" access. Generally the program(s) will disguise themselves, modify part of the operating system, install themselves as drivers, etc. Supposedly, rootkits may have begun as emergency programs to get control of an unresponsive, but are now almost all malware.
• Trojans: No, not that kind at all. Usually a trojan horse appears to be one thing so that user will download, open or run it but it also does one or more things you really would not like - such as delivering a virus, installing a 'backdoor' program, adding your computer to a 'zombie' network for an attack on some other computer - well, the list is fairly long and obnoxious but they can trash your machine. Not good.
• Spyware: Software which is loaded onto your computer without your knowledge and which can do far more bad things than you might expect. Sure it can monitor your activities but it can also change computer settings, install additional software, redirect your browsing, steal revenues from one advertiser and redirect it to another. It can also collect some of your personal information and send it to its momma. These things rarely are alone. Once the get on your box, they may invite in friends. Since these are parasitic processes using your machine without doing you any good, eventually they can make your computer so slow it's unusable.
• Virtual Memory: See the entry for Page File above.
• Virus: Viruses breed. They make lots of copies of themselves and some viruses will create modified versions of themselves. Some viruses will destroy data, damage programs or files, or reformat hard drives. Others will pop up text or display video or audio. They all take up your computer's memory and hard drive space. While usually viruses require that the host file be moved to a new machine to infect it - they do not move or mail themselves, if they get on a network drive accessed by other computers they can spread to new machines. File-sharing and email are probably the main delivery routes. Since viruses are also often poorly coded, the bugs (programming errors) can also cause you to lose data or lead to system crashes.
• Worm: Another type of self-replicating program but unlike a virus it does not need to attach to a host program and it can use a network to send copies of itself - usually without the user having to do anything - to other computers attached to the network. Worms generally cause damage to networks even if all they do is use up network resources such as bandwidth. While some worms do nothing except reproduce and spread, others can and do carry additional code which do a variety of bad things to a computer such as install a backdoor allowing remote control, delete files or encrypt them or send documents by email.

Note: Wikipedia.org was a primary reference for much of the information here and I recommend it as a resource for further information on any of these items.