A couple other simple things you can do to pimp your rig.

1. Create a user account with limited privilages and use that instead of the default admin account.
2. Increase the size of your paging file (swap space). You can do this by:
Control Panel|System|Advanced
Then click on the performance settings, and then the advanced tab. Best bet is to set it to atleast 1.5 or 2 times your ram.
3. While in the performance settings go over to Visual Effects and turn off all that shit.
4. Choose to go with the classic theme, start menu, and window settings.
5. Fire up MSConfig (Run|MSConfig) and click on the "Startup" tab and turn off any shit that doesn't need to run at start up (like MS Office, AIM, shit like that but not your virus scanners and the like.)
6. Run Chkdsk and Defrag (In safe mode) regularly.

Edit: Damn, you slipped in another one. Yeah sometimes there are problems if you have two virus scanners running at the same time. Best bet is to pick one that updates signatures often. Weither or not you want to go with the suite is up to you, but remember that like all software companies some make one product well and the rest can suck. The tools we listed are free though.
"Remember how much fun you had shooting spitwads at the teacher in seventh grade? Imagine applying that kind of attitude to actually fucking with Mitsubishi!"
- Jello Biafra