After several hours, programs begin to fail

I have Windows 7 Home Premium 64 bit, 2 gigs RAM. It was a clean install, not an upgrade. And it worked fine for several weeks.

A few days ago, I began to have a problem. If I leave the computer on for several hours,
the functionality begins to decrease, like available memory is shrinking.
Let me clarify:
if I have 2 or three programs open for awhile, and then try to open another one, it will not open, sometimes saying the program has encountered a problem and has to close, sometimes saying not enough memory, sometimes just failing to open completely. If I close one of the open programs, however, I can usually open the program that wouldn't open before. The longer I have the computer on after this, the more things deteriorate, so that eventual certain functions of open programs begin to drop off. At this point, if I try to open certain windows programs like file manager, I sometimes get something like page cannot be displayed. And eventually, I cannot open Task Manager. At that point I reboot, and am ok for several hours.

I have a vague idea what happened. I was running a program called Advanced System Care Professional, and had selected an option to delete log files I no longer needed. After selecting the ones I wanted to delete, it ran for awhile and froze, and I had to reboot the computer. The problem started after that. Something got screwed up, I think, but I don't know what.

I've run sfc /scannow
I've turned the computer back to a restore point before the above occurred.
I have Acronis True Image Home, and had run a backup image of my computer last week. I restored my MBR from that image. (I haven't yet restored the entire image.....I wanted to see if there was anything else I could do first.)
I've removed a bunch of memory intensive programs from my startup.
I've run the Windows Memory checker which found no problems.
I've run AdAware, and Spybot Search and Destory, and keep my antivirus updated so did a system scan.
I've run a system defrag.

Can anyone direct me what to try next?

Thank you in advance.

Zane

I went back to the earliest system restore available.
The program itself was uninstalled.

I don't know CCleaner....is that a safe program?

I was wondering if, perhaps, something was damaged in my computer's disk caching process. Would this be a possibility, and if so, how would I go about checking something like that?

Thanks for your response.

Zane