Wedge,
The reason I mentioned a pest is because a fresh install of WinXP would not have all the security patches and if you went on to the internet without being behind a firewall in order to get the updates, you could have been infected. Second, if the pest was in your data, when you put you old data back on to the comp, you could have been infected there again.
Now, since this could be mouse related, I would be interested to see what your bare bones test comes back with.
Take Care,
Richard