This is the stuff i tend to do when my PC is running slower than it should.. but i run on XP so some of it may be different for vista.. havnt got round to tryin any of it on my laptop which runs vista just yet >.<

1. De-frag you're Computer
- Start > All Programs > Accessories > System Tools > Disc Defragmentor

2. Perform a Disc Clean-up
- Start > All Programs > Accessories > System Tools > Disc Cleanup

3. Perform Error Checking
- Right Click "C" Drive (Or Whichever is you're Main Drive) > Properties > Tools(Tab) > Error Checking

4. Clean-Up Registry
- I Use a Program Called "CleanMyPc Registry Cleaner" For this.

5. Uninstall Unwanted Software/Programs
- Start > Control Panel > Add or Remove Programs

6. Check for Adware/Spyware & Update Anti-Virus/Spyware Software
- These can majorly slow you're system, so having good Anti-virus/Anti-Spyware software is always a good idea.

7. Make Sure All the Windows Updates are done
- I know.. a pain to keep installing every little update windows make, but it's worth it if you want to keep you're computer up to date and "Healthy" ^^

8. Configure Start-Up Programs (This can really help speed up the systems booting process...aka.. Switching it on! ^^)
- I Use "CleanMyPc Registry Cleaner" for this since its easier, but you can do it without.
- Start > Run > Type "Msconfig" And Hit Enter > Startup(Tab) > Uncheck Programs you don't wont start when windows boots up.

9. Clean up you're Internet Browers (Not Necessary for Speeding up you're entire computer, but it does speed up browsing alittle so i do it anyway ^^)
- I use "CleanMyPc Registry Cleaner" for this also.. very handy software i am noticing lol but obviously it can be done without it.
- Start > Control Panel > Internet Options > Delete(Under Browsing History) > Make Sure "Temporary Internet Files" "Cookies" And "History" are Checked > Click Delete

10. Since you're Using Vista it might be worth mentioning "ReadyBoost", it's a new way to add memory to you're system by using a Flash Drive, So if you have one lying around it might be worth trying that, just a thought, i havent tried it myself yet.. so i can't really comment too much ^^

I think that's all i can think of atm..i'll let you know if anything else comes to mind, hope that helped anyway ^^