I don't know about Windows XP sorry. I have Vista on notebook and 2000 prof on stationary PC.
I had some problems with deleting indicators and placing indicators in MetaTrader's folders on Vista but it was disscussed on this thread so I know about it now and moved your post because of that.
I don't know anything about Windows XP sorry.
By the way I like Windows 95 more than XP and Vista
Try to de-install/install Metatrader. Or try universal decision: restart computer.