My PC (with Windows XP) is off the rails!

The past week most Google and Yahoo search results would redirect me to various sites, not the search result site. I used Spybot S&D, MalwareBytes and AVG 8.5, which usually found and quarantined various problems, including WIN32.TDSS.NTF, but they didn't correct the underlying problem. Search results are still redirected. Perhaps related, when I boot up I get a pop-up saying "Your System has no Paging File, or the Paging File is too small."

I downloaded ComboFix (have NOT run it), HiJackThis, AdAware, as well as IE 8 and some Windows patches. I also tried to update my Java, but am not sure where that stands.

NOW, Antivirus Pro windows keep popping up, and I can't launch any of the above programs including HiJackThis. Each time, Windows asks me what program I want to use to open them, instead of launching the program.
Any help would be greatly appreciated!!