I finally got HOTU and installed it.

I tried to finish playing my SOU game and it says it is unable to load because it was created on a different toolset/game version. I have NWN gold and bought HOTU separately (I actually found it after the Platinum version was released!). When I put the NWN gold play disk in to try and play my other game, it says to use the HOTU CD.

I run Win XP and previous to installing HOTU, I had NWN patched to version 1.64.

I hope these games aren't lost because I worked VERY hard to get where I am in them.

Did you upgrade HotU as well? I think HotU was shipped with version 1.62 or it might even have been 1.59. So see if you still have the proper updates.

You wont be able to use the previous updates even if you saved them, which you probably didnt. There are different patches for each expansion. Run the updater and it should bring you up to 1.65, which will run saves from 1.64. If the updater decides not to work for some reason, you can get the patch manauly from here:
http://nwn.bioware.com/support/patch.html