make sure your anti virus is up to date on your machine and make sure you are doing regular scans of your computer.
i would also aquire the following software to help cleanup your system:
The home of Spybot-S&D!
defrag your harddrive and perform error checking (right click on local disk c and click on tools)
also check how many items are running in task manager around 30 is about normal.
check msconfig (start> run > "msconfig" > startup tab) make sure you check what programmes you want to startup when you first start your machine up. On my startup i leave my anti virus programme to run on startup and remove everything else. This will make your machine faster and may stop it from randomley crashing/freezing.