Get yourself the following tools...
CCleaner
This tool will clean out all crap that gets left behind in the various temporary file areas. Files, cookies, cached pages, etc. It also can help cleanup the Windows Registry as well.
You can find it here...
CCleaner
avast!
This is a great free anti-virus tool that that costs nothing to to subscribe to updates for its virus database.
You can find it here...
avast! free anti-virus software
Revo Uninstaller
This tool will uninstall pretty much remove any program you have installed in your system. It run the programs own uninstall and then scan the system for anything that may be left over. It is quite thorough.
You can find it here...
Revo Uninstaller
Microsoft Security Essentials
Yes, even Microsoft has made available a free anti-virus tool. It is also quite good and will cost you nothing to keep its virus database updated.
You can find it here...
Microsoft Security Essentials
Windows Defender
This is the firewall that comes with Windows. It was first introduced in Service Pack 2 of Windows XP. You need to have some kind of firewall up to protect the malicious stuff from getting in.