Is this with SoulseekQt or with SoulseekNS?
SoulseekQT. i have reinstalled older builds where this wasn't a problem, yet it remains. SoulseekNS works fine though.
See if you can find a file called soulseek-client.dat on your computer, it should be in your user application data folder (On Windows 7 that's c:\Users\[username]\AppData\Local). If you can't find it I recommend Search Everything. This is your client's configuration data file. I suspect the window position information went out of bounds somehow and would like to take a look at it if you don't mind. There's a Send File link on my user page you can use for that.
Aye cheers, sent that on to ya there, I'm gonna try reinstalling after deleting that file and see how that works out for, had a suspicion it was something to do with that alright.