running out of ideas to stop my computer slowing down.

[SIM-O]

My computer is slowing down again, and like most times I've had a computer, i don't really know what best to do. I have Bitdefender total security, which according to this page: AntiVirus Software Review 2009 - TopTenREVIEWS is up there as one of the best.
I've tried
-defrag
-deep system scan for viruses, cookies etc
-malware search (malwarebytes' anti malware)
-temporary file cleanup
-clean up
-clean registry
-my fire wall is up
-my antivirus setting is set to aggressive
-antispam

over the time i got my computer, it's slowed down considerably. and I'm pretty sure it's not my bitdefender, because it's just gradually slowing down.
it took twice if not 3 maybe four times as long on the desktop for everything to finish loading.
what do people do when that happens?? I'd like to get this down once and for all.

Edit: oh and if it helps, i get this error message on startup:
Error loading C:\WINDOWS\system32\vapudabi.dll
this specified module could not be found.

I did my research, and it turns out that file is unsafe. Since i can't find that file in it's location, and neither can bitdefender, i assume that the only reason that message is coming up, is because another unwanted file is trying to run it. maybe someone knows how i can track that program down.

[Need_Help]

If I were you, I will use other anti-virus programs as bitdefender is using a lot of system resource (ESET or Norton are way better than this, don't fully trust this kind of reviews).

Besides, I would advise you to backup your files and do a complete format as troubleshooting which file has caused this will take longer and may not get the expected result.

Thanks.

[SIM-O]

can't i do it without reinstalling everything and having to revert all my settings?
(at least that's what i thought a format did) I would have thought doing something like that could take ages to clean up after.

[SIM-O]

well i just used 'speedupmypc 2009' and my computer is much faster. i also went to 'msconfig' and sorted out that little problem with the mystery startup attempt to find that dll. i also found a few other programs that were running...when they shouldn't have been. so now my computer is going much faster, but i still think I've got some things to do. I'd rather like to try other things before i start over...