can you stay in task manager long enough to see if there are any suspicious processes running ? something that could cause a system to be unstable ? ....even if its an antivirus software, it might be causing all this trouble...i would end all the processes you have running, except the ones that need to be there for o/s to function - and go from there....
you could also try running: msconfig from the run command if you are able to get there, to see what programs start when windows starts, and turn them off...you are also able to run system restore directly from msconfig
from msconfig you can also boot into safe mode with custom parimeters if you are interested....its a very useful utility - startup tab will also list the programs that start when windows loads up - check that if you can