Is it only one one particular project or does it also happen on a new project? If just the one then you may have to recreate it from scratch and copy the code in to the new code window. If that doesn't work you may have to uninstall/reinstall Visual Studio.
So it's on Windows XP? I have a buddy that their XP OS was so banged up over the years that they finally had to start over. There are those that say never say die but how many weeks did he lose over that?
Perhaps a tad too late (maybe not) but I suggest you partition your drive into C (60-80 gig OS and apps) and D (whatever is left) for data and relocate your user folders (My Documents, My Pictures, etc) there. Install your OS and apps, apply all updates, delete all temporary files (if you applied any Windows service packs then do a Freeze). Install the free version of Macrium Reflect and take an image of C. That way you'll have a clean system to go back to without having to reinstall everything again.
A Freeze deletes all the service pack undo files (frees up a lot of space). To do this, copy the following code into FREEZE.CMD
DISM /online /Cleanup-Image /SpSuperseded
The only catch is that you have to run it as Administrator. The two Time commands are so you can see how long it took and the two dir commands are so you can compare the free space before and after. The whole process can take up to 30 minutes or so. Be patient.