I really like
WinASO's Registry Optimizer program. Only lets you repair 10 faults on each scan, but there's no limit to the number of scans.
Download, and install,
Lavasoft Ad-Aware and
AVG Anti-Virus. Once you've installed both, open them and install updates.
Then, once all done, DISCONNECT from the internet, run a full scan using the Ad-Aware program first. Then, run a full scan using AVG Free Edition. Then, run the WinASO Registry Optimizer until it doesnt pick up any more problems.
You're pretty much all done. To clean up the little bits here and there, use other anti-adware and anti-virus programs to get what the first programs missed.
But wait, there's even more you can do. Start>Run>type 'msconfig'>click ok> click the Startup tab (on far right)>unselect EVERYTHING, then go through and reactivate firewall, and your internet connector (no need to if you're on a router). Click anything else you want to open up every time you turn computer on. Remember, the more you select, the longer it will take to turn on.
Control Panel>Add/Remove Program>Uninstall stuff you KNOW you dont want. Don't uninstall programs unless you know what it is and you definately want it to go.
Finally, Start>All Programs>Accessories>System Tools>Disk Defragmenter. Run this overnight, or when the family goes down to the beach. It can take quite a few hours.
Someone should make a sticky, titled 'Detox for your computer'. It'd be so much easier.
