Delete all your other security programs as most malware circumvent them installing properly and infect the programs directly.
Does the computer have a legit version of windows on it - if not your knackered. If you have not updated your pirate version of windows since windows 2000 you'll get a virus in seconds just by plugging into the web.
I know its a swear word - but microsoft - install
http://www.microsoft.com/en-gb/security_essentials/default.aspx - you'll need a correct windows license for this.
After doing a complete scan, change all your passwords to nothing obvious - try reversing a telephone number you know from childhood and combining it with a street address - the longer the better
It is possible for a computer to be so infected its impossible to fix, do you have a restore disk and a means of posting all your pictures and information via google to yourself? (they scan email)
Ta
Baz