Quad core with three cores doing nothing

Hello,

I have an Athlon Phenom X4 925 and installed windows 7 32 bit on it about 2 months ago. Everything was working great. Applications were assigned to different cores and TMpgEnc used all 4 cores when encoding.
But today I noticed that TMpgEnc was slower and I saw in the task manager that only the first core was used. With that core at 100% all newly started applications were assinged to that core also (I saw that the affinity is set to "processor 0" for all processes). When I set the affinity for TMpgEnc to "all processors" it starts using all 4 again.
Is there a way so that windows does this automatically again? Using alle cores for all processes?

Thank you.

Do you mean Boot->advanced->number of processors? I'd already tried that but didn't help (is set to 4).

A system restore didn't help and a repair install won't guarantee that it will be fixed or return again in a few weeks (this already happened within 2 months of using windows 7). I also tried disabling all non-windows services and start up programs with msconfig. It made no difference but I know now they aren't the cause of it.
I found Bill2's Process manager, a great tool to set the affinity to 1 or more processors and it remembers it. I'll stick to that until I find a solution and/or the cause. When I do I let you known.

I found the cause for this problem. When I started using windows 7 I had zoomtext 9.18 installed to see if it was working ok with W7. Everything was working good back then. But because the trial period stopped prematurely I had to switch back to zoomtest 9.14. Turned out that it forced all programs to use core 0 because it gets unstable when programs are scattered over different cores.
I hope to be able to install version 9.18 soon again so everything will be back to normal.