Dennis,

You need to determine the cause first. So far you are only seeing the result: it keeps rebooting without finishing. You need to do this and get the Stop Codes: as you turn on the PC, start pressing the F8 key repeatedly right away (on some PCs you have to wait till immediately after the video BIOS or System BIOS splash screens are done, but before Windows starts to load). This is often easy, but not always.

When you succeed, you will get an expanded selection of options (Safe Mode, etc.). Choose the one that says "Do not restart on failure ..." (or something similar). You will then get the infamous Blue Screen of Death (BSOD). That's good. There will be an error code listed, and perhaps additional info. The error codes are in this format:
Windows Stop Code 0x00000000 (0x00000000, 0x00000000, 0x00000000).
There are always 8 characters after the 0x. The leading zero's preceding the first non-zero character are place holders. The most important of these codes is the first set. The codes in parentheses are sub-codes that can sometimes help, but usually are not needed to solve the problem. So if the codes were 0x000000EA (0x00001234 0x000095C7 0x000000BD), all you should need to research would be the 0x000000EA Code.

Sometimes you also see text near the top that looks like this: PAGE_FAULT_IN_NONPAGED_AREA. If you see some message like that as well, write it down. The last clue you might find will be if it tells you a particular file that is having the problem. Take note of that as well.

Now here's a good web site that summarizes the Stop Codes and often points to solutions: http://www.aumha.org/a/stop.php

This should get you going. Good luck!

Chuck