Help!!!

I have a user control which is a textbox with a couple of extra features, like background colour change on focus, number formatting etc. All this works fine, however, my machine died and instead on putting back windows 2000 prof I went for XP. I've reloaded everything and put my user control back onto my toolbox. As soon as I drag the user control onto the form I get the message 'Object Reference not set to an Instance of an Object'.

I've tried recompiling the user control and removing it and re adding it to the toolbox but nothing I try seems to work. If I reload an old project the user control does not show on the form. I've also tried removing and re adding the reference as well.

Any ideas?