As long as you haven't made significant changes to your hardware, you shouldn't have any problems. I've reinstalled more times than that in a month, and never have had any issues. Is it asking you to call Microsoft? If it is, it should be just a matter of calling the 1-800 number and saying "I had to reinstall x amount of times because I was .....) and they should reactivate it for you.
Unless it says you have to re-activate it don't worry. It may still be activated from the last install. All they do is drop a file on the disk and make a regsitry entry. So if the re-install did not change hardware or delete the file you're still active.
If it is asking for a re-activation but won't let you over the net call the 1-800 number as above.