Packard Bell or HP? Confusing.
The easiest way is to contact them and ask for a restore CD. Otherwise you need a CD with that same version of XP (pre-SP2, probably) that accepts the product-ID from the sticker. A wild guess.
I would advice you to download a free program called TREESIZE to find out where all that diskspace is being used. It might be you can just free it. Also try CCLEANER to delete a lot of junk, and disable system restore (enable immediately after).
Hope this helps, post again if necessary.
Our Packard Bell was bought about 3 years ago and never came with ANY CDs - XP etc. was already installed. Now we are having problems - there are hardly any programs or files installed on this computer yet it is saying almost all of the 40GB HDD is full!! I am thinking this is due to a virus (I recently removed about 140 infected files!!) I was thinking about just formatting HDD / reinstalling Windows but how on earth do I do it? (The authentic product key sticker is attached to the tower)