I think I've done something to my computer, now all my files end with their file type (eg .mpg) and I can't change the file name. Can someone please help me out?

In an explorer window like My Documents;

>Click Tools
>Click Folder Options
>Click the View Tab
>Check "hide extentions for known filetypes"
>Click Apply then OK

Things should be back to normal.

It worked! thanks, bh. I remember now it was when I was trying to delete some adware. God I hate that stuff.