My Windows XP (SP3) machine (at work) occasionally "forgets" how to cut-and-paste files in Explorer - if I cut or copy a file then go to paste it elsewhere, the "paste" and "paste shortcut" menu items are greyed out.
The keyboard shortcuts (Ctrl+X/Ctrl+C,Ctrl+V) don't work either, and neither does Drag-and-Drop - clicking and dragging a file/folder doesn't change the pointer and letting go doesn't drop the item anywhere.
Restarting the explorer.exe process usually fixes the problem, but sometimes I need to do a full reboot.
Note: this may be related to the following existing question, although that's Vista and it sounds like the asker has more permanent problems than I do:
So, any idea what might be causing the problem and how to avoid it?