Although the Linux version of GAIM (a multi-protocol IM client) has always worked well for me, the Windows port I found too buggy and unstable in the past. This may have been the fault of the Windows port of GTK+ which seems to have improved recently. Recent versions of Gaim for Windows have been running smoothly.
I ditched the official clients and Trillian Basic and am using Gaim for all IM. Though Trillian is also much better than it used to be. I found previous versions to be buggy and the UI to be poorly designed (I tried many different skins without finding one that helped this). Personally, I think the use of bitmapped (skinnable) interfaces for an IM client is not the best idea, but the current version of Trillian Basic has an excellent default skin and is stable. I actually would've kept using Trillian, but the lack of Jabber support in the free version made me switch to Gaim. My only complaint with Gaim is that I can't individually enable/disable logging for individual accounts. I can either have logging enabled for all accounts, or not at all. This was also a problem with Trillian Basic.
Speaking of GTK+, GIMP for Windows has also been running relatively smoothly. For those unfamiliar, GIMP is a free open source image editor. GTK was actually created for GIMP and in fact stands for "Gimp Toolkit."