Hi i am new to win7 but what i would do first is check if your CPU use is say at 100% when this is happening, if it is, open task manager (ctrl-alt-delete at the same time) and see if a program is using a lot of CPU.
I had this 1 or 2 times with XP , 1 time it was some free anti virus software, PC doctor i think and even when it was not scanning it was using 100% CPU so the computer was slow and lagging when i kill it in task manager all back to normal, so i get rid of PCdoctor hope this help
s Dave