Icons change on their own

I don't know what I've done to my computer. Every now and then all files having a particular extension and icon take on a different icon, for example, I've just been working with a spreadsheet and when I closed it I noticed all my spreadsheets (*.xls) are now listed with a new icon representing 2 little computers.

I can change this manually or reboot the computer. Still, is there a reason for this strange behaviour?

Are you using windows 98/SE? If so then it's a bug in windows.

This usually happens when uninstalling some programs which used to run these icons .