Applications won't start

My computer seems to have a "lazy mode". Some of the time it's fine, but sometimes when I start an application (it can happen for any application, but seems to be more likely for ones that use more memory) it stalls mid-startup, or slows down to the point where its are effectively stalled. The program will show up in the "processes" tab of task manager, with the CPU hovering around 50% and memory usage increasing veeery slowly. So it's doing *something*. The application doesn't show up in the "applications" tab of task manager, only in "processes". If I leave it going it can take up to half and hour to start, and then runs with heavy performance issues. (If it takes longer than that, I've either rebooted or stopped the process.) It's as if there's some huge resource hog going in the background, but nothing shows up in TM (showing processes from all users).

This doesn't happen all the time. Sometimes applications will start fine and run without a problem, but often they won't. And once applications have stopped loading I have to reboot to get the machine out of lazy mode. Sometimes lazy mode takes a while to kick in, but sometimes it's there as soon as I boot up.

I have not been able to recreate this problem while superfetch is disabled. Which isn't to say that superfetch is the cause, but disabling it does seem to aleviate the symptoms. Obviously, I'd rather have superfetch running.

While my computer is in lazy mode, its winsat results drop considerably. For example, winsat mem goes from about 4600 to 3300 or lower, and the first of the cpuformal tests can't complete because the machine is too slow. I have and Intel Core 2 cpu 6600 @ 2.4GHz and 4GB of RAM... which I discovered yesterday from Crucial.com is two 1GB DDR2 PC2-5200 and two 1GB DDR PC2-6400 (does a 4600 rating seem low for that? 3300? The 6400s are new. Are they compatible?).

I've put pagefile on a separate hard drive, if that has anything to do with anything... but I suspect not because I've never seen it get used in performance monitor even with multiple applications running.

I try to keep it clean, with daily ESET virus, Malwarebytes and IOBit malware scans. All of which come up clean (except for InstallCore.C from SIW which ESET picks up, but I'm assuming that's a false positive). So as far as I can tell it's not malware or a process running in the background. sfc /scannow also doesn't pick up anything, either.

The only other thing I can think of besides iffy RAM is that the IObit registry fix "fixed" something it shouldn't have.

It's a new OS install from a couple of weeks ago when I replaced my hard drives and had the RAM installed. I'm kind of paranoid that the guy who did it had no idea what he was doing (he didn't know how to set the pagefile, for example) and somehow messed it up. So reinstalling Windows isn't beyond the realms of possibility, but I'd really rather not.

Any help would be massively appreciated!