Copy the P99 folder to a flash drive or burn it to a disc.
Remove your client entierly from your machine.
Do a complete fresh install don't use the default folder in the insaller make it something like c:\THF. (Easier to put it on the root directory for spell file updates

)
Copy your P99 back make short cuts that point to the appropriate folder you want to access.
If you can't do this try what I did.
I have three instances of my client on my hard drive right now each folder is 3.84 gigs(need to fill up that 2TB some how). I run some toons with different key and chat color set ups and one points to the login public server. When I first installed my client I installed my master copy on my server. Then I copy and pasted from there so I could customize each folder how I wanted it. This way you always have a fresh unpatched (be the patch THF or another server) copy of yor client. Your client will run with out the installer running on your machine so you will then be able to patch as necessary for the server you wish to play on.