Attempted to access invalid adress

That's the message I get when I try to start some program installers. I've tried virus scanners and all sorts of other bad-things-removers but I just can't figure it out. I'm running XP MCE with Intel Pentium 4 and have 1.87GB available RAM with around 130GB of HD space. A long time ago something big attacked my computer, I don't remember what but it took hours to get rid of. I know it attacked the registry ever since then(Months ago.) I haven't been able to use some installers and task manager won't open. Any ideas or needed information?