I tend to leave my machines on for days or weeks on end. I've recently switched to Vista full time and that switch started with a new PC I bought for the lounge. I noticed that after being left on for around 24-36 hours the task bar would go all transparent or white and the task bar buttons would cram into the left hand side. There was other oddness going on such as the start menu getting corrupted. At first I thought it was a fault with the graphics card (sinking feeling) or possibly the driver but a reboot would make the problem go away for another 24-36 hours and then it would come back. This smelled of a resource leak somewhere because there was a reasonably identifiable trend in time and the fault manifesting itself.

I googled around for a bit and discovered that there's a known problem with a GDI object leak in ComCtl32.dll. More about it here and how to fix -

The Windows desktop may stop updating correctly after a Windows Vista-based computer has been running for an extended period of time (http://support.microsoft.com/kb/932406/en-us)