Can't launch multiplayer

Hello, for a while I’ve been having the off-and-on problem of Gmod crashing whenever I try joining a multiplayer server, spitting out an error box.

Any ideas/suggestions?

edit: It would probly be good to give system specs…
Phenon II x4 955
nVidia GeForce GT 520
Windows XP 64-bit

There’s your problem. Your CPU is pretty good, modern amount of RAM, cheap HD graphics card yet it still definitely has to run GMOD.
You do realize that XP no longer gets any proper support from majority of developers. XP is complete garbage, all it does today is “Oh sorry I crashed, should I send or don’t send this crash report to completely automated server that nobody is going ever to look at”.

Switch to W7 when you can as there is completely no reason why you should limit yourself to XP features, or try re-installing software randomly and hope it fixes something like Video drivers, update Direct X, Frameworks and so on.

My OS isn’t the problem, the crashing seems to happen off and on, no matter the OS.