Dr Watson Postmortem debugger :S:S:S

Sorry if it's in the wrong board, I didnt know which to put it in.
But Lately, I've been getting this error, Called Dr Watson Postmortem Debugger. It comes up when i'm running a couple of things and they freeze. It comes up, and I click Don't Send. After this occurs, I can't open any window, and I can only use the window open.
For example, one of the times, I was on MSN, While I had gamespot, steam, winrar, and a couple folders open. My Winrar extraction completed and i opened up a conversation to tell a friend it was done. My comp lagged a bit and the error message came up, and I clicked Dont Send. It colses but every other window except the last window open (my conversation with my friend) was malfunctioning.
I've noticed this happens when I'm installing mods for Garrysmod.
I Just turn off my computer and restart it to get by it.
Does anyone know what this error is, and how I can fix it? It's beenh appening alot lately, and I don't know what to do.
thnx!