I run a jalopy of a system, and it simply can’t handle XP, let alone Vista. I don’t have the time for the learning curve of linux (the PC I’m writing this on is old enough that only older distros work well with it). So the IM client of choice is Miranda IM. It’s extremely low-resource and versatile… And that worked GREAT with Twitter… until IM support went away.
There are some command line sending methods, which are fine… but no automatically refreshing windows. There’s supposedly a plugin that is in the works, but until then, you can use RSS News to read your timeline. Since it supports authentication, you’re golden. No avatars or sending capability through it, but that’s at least *something* for your IM client…
 I was having trouble finding the IMified bots; while you can sign up at thier main page, the bots themselves are at the (improperly linked) old site. Their twitter bot is exactly the opposite; it will send but not receive.
Using TabSRMM makes it not too painful.