Schizotypic, if I were you, I'd do the following :
- Uninstall every program you don't need. I would also suggest uninstalling your antivirus because a lot of these virus programs are heavy in program requirements (examples Norton, McAfee) and I'll suggest one which is free, good and a little more easy on the requirements in a bit.
- Remove everything unnecessary from running at startup. In Vista, you have control over what starts with Windows. In XP, it only takes a sec to get Startup Control Panel, a handy little tool that gives a "startup icon" on your control panel that you can click. Then you can choose not to run stuff.
- Clean out all spyware. I see you've gotten some suggestions that you could use. I use Spyware Doctor myself and it's an excellent program. However, it's not freeware.
- Clean out and compact the registry. I would recommend the program Registry Mechanic made by the same company behind spyware doctor. All the programs you've had on your computer are gonna leave stuff in the registry and these are not always cleaned out, even after uninstallation.
- Get avast! Home Edition and install that. It's an antivirus program and it's developers sell it to businesses. For home users, however, it's free, easy on the reqs and it's also quite good.
- Do a thorough search for viruses on your computer
- Defragment it over night
Alternatively, if you're worried about getting viruses in the space of time between uninstalling your virus protection and installing the new one, just put those two events after one another ..
Doing these steps will help
edit :
Writing this up reminded me that I should do a little spring cleaning myself. I noticed that the last version of registry mechanic has something called "registry monitor" which basically watches your registry and let's you know if anything tries to do changes with it. I don't like unnecessary processes running in the background and this was easy to turn off!