if restarting the client don't work its most likely a background application or windows background tasks playing with you, i would suggest keeping an eye on system resources when/if it happens again.
ctrl + alt + delete
task manager - processes/performance tabs
My cpu usage, memory usage, bandwidth usage and system temperatures are always visible to me thanks to the lcd screen on my G15 keyboard and LCDSirReal. When I am having this problem memory usage is normal, and cpu load as well, and I'm not noticing any other funky hardware issues, and I've got plenty of hard drive space on both of my drives and all the partitions. I've also checked the task manager and nothing in the list is tying up any exceptional amount of resources.
It's a mystery Charlie Brown. The only thing I can think of is something like the EC in Windows prefetch becomes corrupted, but I looked at that and it doesn't make any sense to me. Also if it was a prefetch issue you would think that rebooting wouldn't fix it.