Get ad aware from
http://www.lavasoftusa.com/
Run this programme a few times to get all the shit off your PC
Then get Google toolbar with pop up blocker
http://toolbar.google.com/
Then get free zone alarm from
www.zonelabs.com
Then get a good antivirus programme.
Always have Zone Alarm and your antivirus running in background.
Have automatic update set (for XP)
Try this and see if it helps
If you don't have it get Windows XP as it's more secure.
It doesn't matter what protection you have if someone has physical access to your PC they can get in. It takes 20 seconds to get into Windows XP Pro. Inserting a Windows 2000 CD into an XP system allows one to bypass all password protection and manipulate any part of the machine at will. Anyone with a Windows 2000 CD can boot up a Windows XP box and start the Windows 2000 Recovery Console. The intruder has Administrator privileges even if he or she does not provide a password, and can also assume the identity of any other user of the machine.
In Microsofts own words "If someone has unrestricted physical access to your computer, it's not your computer anymore."