If you've tried all the anti-virus/spyware/adware software out there and still keep getting infected, then you might consider using a radical alternative. Sandboxie basically acts as a middleman between your PC and any programs and applications that are running on it and offers an interesting alternative to traditional anti-malware apps.
When you run a program on your computer, data flows from the hard disk to the program via read operations. The data is then processed and displayed, and finally flows back from the progam to the hard disk via write operations. Sandboxie changes the rules such that write operations do not make it back to your hard disk. This doesn't affect the operation of your programs in anyway but crucially, it prevents any nasty programs from leaving unwelcome gifts on your PC.
It's particularly effective against the dangers of ActiveX. The ActiveX script allows websites to run small programs on your computer. They are mainly harmless but some use them to install spyware on your computer. Normally, your best option is to browse with ActiveX disabled (by turning it off, or by switching to a browser that doesn't offer support for ActiveX), but in doing so, you would be trading security over functionality. With Sandboxie, you can keep ActiveX turned on, and have both security and functionality.