I always loved Warcraft 2, but can't get it to run on WinXP well.
What other similar games are out there avail on gog?

Aren't the versions sold at Blizzard's store supposed to have improved compatibility with modern OS? Not that I'm saying you should rush out any buy another copy. But it might be worth looking into.

Blizzard doesn't sell any pre-Starcraft games at their store.

Well, there goes that theory.

Umm.... Maybe Earth 2140?

I haven't actually played it yet, but from descriptions, Warlords Battlecry 3 seems rather close to WC3.

I haven't played warcraft 2 yet, but i think Warlords battlecry 3 is what you're looking for.

Try the Battle.Net edition of Warcraft 2. I'm sure that one runs on XP (at least it did for me few years ago).

Run the DOS ver in DOSBox
Run Battle.net in Windows

Did you try the "Battle.net Edition", you should be able to find it for sale somewhere. If you aren't stuck on the fantasy aspect of the RTS Empire Earth is halfway decent (mandatory issue, I've found no way to change the resolution in the original, this might bother you - it may still be possible by editing a config file, but I have not found it).

Try the free combat edition. Theres no campaign in it however, but skirmish and multiplayer is free. http://server.war2.ru/modules/news/

It does. I have it installed right now (and I run Win XP).

I also have the Battle.Net edition and it works fine under XP.

Answering th eoriginal question - Warlords: Battlecry 3 is pretty good.