If you have a stop error, you need to know which one you have - they're numbered, like STOP 0x0000007A
If it keeps restarting pretty quickly (a few seconds), the most likely problem is the CPU fan - it probably isn't turning. The CPU overheats almost immediately, shuts off, the computer is set to restart on power failure so it restarts, the CPU overheats, etc.
If it's slower than that, you could have another fan bad, or the air vents could be clogged.
Dot net is a program library. Programs written to use that library (there are 4 of them - 1.1, 2, 3 and 3.5) will give you an error (that you need the dot net framework). Deleting it doesn't cure problems and, as long as you can download the missing one, won't cause any problems.
As far as not having genuine Windows, a call to Microsoft will clear that up. But I suspect that you have one or more viruses on that computer that you have to clear up first.