That would be your Desktop Manager going skitzy on you. The same thing is happening to me, but i can't figure out why...
The only solution i could think of was to create a .bat file with the commands "net stop uxsms", then something like ipconfig to lengthen the time between commands, then "net start uxsms" to restart the process. This will end the bugged running dwm.exe and then start a new one. This really isn't the solution, I could propably pull more hypotheseis if you'd be kind enough to list your specs in case its something like hardware we have in common.... same to any readers out there with this proplem.
2.8 duo
4gbs ddr2 pc2 5300
8600GT 512mb
160gb, 250gb SATA2 drives
windows 7 build 7068
overheating all the damn time
one optical DVDRAM+- ,ect all of em' ya