PC freezing, shutting down etc

I have a Dell Inspiron 1545 running Windows 7 64 bit with 2.16 GHZ of processor and 3 GB ram, it has been very slow recently and now every minute or so the screen blinks out flashes a few times (no error message) and turns back on except that the areo effects are off for a minute or so, I am able to simulate the effect by shutting down the most memory hogging svchost.exe process which is on average using 87k to 200k.

The same thing is happening to my Inspiron E1505 which is running Vista X86 that has a 1.6 GHZ or so processor and 2 GB of ram, it is slightly more extreme on that one (lasts longer, completely shuts down the screen for a minute, much slower...)

I have used a few malware, adware and virus scanners and nothing came up, neither of them can run sfc in the command prompt. (they shut down or completely freeze up till I shut them down cold)

Any help would be appreciated.

Thank you.

The problem started on the Vista PC first then the Windows 7 machine, I have tried McAfee, Spyware Doctor and SpyBot Search and Destroy (which has worked on everything so far) and I am going to do MalwareBytes, SFC only runs in elevated command prompt so yes I did.

Thank you for your help.

I would rather not do a restore yet because this is a heavily used computer and is changed very often, I am leaving that as a last resort.

MalwareBytes is running now and I will give the results soon.

Once again thank you.

Nothing significant on MalwareBytes I removed the adware it found.
All drivers that I know of are up to date so...

Again, System Restore is going to be your best bet. You say the computer is changed a lot...what do you mean? This very well could be the source of the problems.