I had this problem and fixed it so hopefully someone else can benefit from what I learned.
It started happening after I downloaded and installed some massive crappy HP printer driver software but I'm sure it could be caused by other downloads etc. that require .NET framework so read on.
The first thing you must do is remove .NET framework. You have to remove the 'extended' version 1st and then remove the other one after that. DON'T do it the other way around or it won't allow you to remove the extended one and then you'll be up s^%t creek.
After it's removed, go to this link below and download the appropriate file for your version of Windows. In my case, I'm running XP so I downloaded the "Windows XP SP 1", restarted and the stupid error message FINALLY disappeared!
If that doesn't work, just think it through. I'm pretty sure what caused this in my case was when I downloaded the HP printer software, it installed too high of a version of .NET framework, like version 4.0 so you have to remove it and start with a lower or 'older' version of .NET Framework. The reason I suspect this is you have to remove the higher versions first if you're removing them so I don't think you can just go installing the higher ones willy-nilly without intsalling the lower ones first, you get the idea.
Anyhow, good luck man, hope it helps.