If you haven't done it already, I'd be inclined to take out the hard drive that's in there, and put in another one (they're pretty cheap on Ebay these days). Then, install to that. If it's no better, you haven't lost all your old settings and won't spend days putting them back.
If that doesn't fix it, then put the old HD back as master and make the new one "slave" - so you've got oodles more hard drive space.
If it fixes it, put the old one back as slave (not the boot drive) and you've got all your files back without needing to do vast amounts of copying.
Before you do any of that, run a proper virus and malware scan on the existing HD - AVG plus SpyBot etc (see the sticky at the top of this forum).