It could be that the page file has become fragmented. That can slow things down appreciably.
A quick check to see if that's a contributing factor would be to remove the page file from C and put it on D. I tend to create a fixed page file with a size that is twice the physical RAM, rather than leave it as system managed.
I used to do similar and fix the page file size to the "Recommended" size rather than have Windows manage it, as the recommendation from Microsoft was that it'd never use more than the recommended size and a dynamically varying pagefile would promote the fragmentation of your hard drive.
Now though, after spending the last couple of days poring over page file documents for Server 2008, I can safely say that the left hand of Microsoft doesn't always know what the right hand is doing, and that you need to check the page commit figures and total up a bunch of memory figures before you know roughly what's best for it......and even then MS throws a curveball.
/offtopic but interesting.