Delete everything related to Unum (thus any old wallets including the appdata folders it creates (make sure you can see hidden folders) under C/Usernamehere/Appdata/Roaming/Unum.
Then re-download new wallet and run it as administrator.
Also not trying to sound like a arrogant .... but you really should upgrade if not to windows 8 atleast to windows 7.
Note, I ain't saying you should pay Lamesoft for it, just pirate dat shit