Well, if you had an original Windows installation disc then you could perform what's called a "repair installation" which just installs Windows on top of itself, like you're describing. But you MUST have an original installation disc to do that.
With only a Dell factory disc your only option, if possible, would be to save your data and attempt a factory restore from the disc.
However, do need to tell you that I'm not at all confident that that would resolve your issue. You said that sometimes the machine shuts down shortly after Windows loads. This does not point to a problem with Windows, but rather it indicates a hardware problem. Just based on your description, the problem could be any of the following:
1. The system is overheating due to a faulty fan or heatsink
2. The motherboard is damaged
3. The RAM or CPU is faulty
Systems randomly shutting down are almost never due to a software issue and are almost always due to a hardware problem.
Considering that you are running Vista, I have to assume that this is an older computer. In that case, I would not recommend investing money into hardware repairs for the computer. I would instead put that money into a new machine.
If you have data on this computer that you cannot afford to lose, and the machine will not stay on long enough for you to get the data copied over to something like an external hard drive, then you can remove the drive and connect it to another computer and copy the data over.
I hope all this makes sense. Let me know what further questions you may have on this.
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