thank you both for your suggestions and interest in my low hd space issue. .
my way for safety is, I create 2 partitions, keep windows on 1st partition and all my personal data on 2nd partition. after a clean windows installation I also install all my necessary 3rd party software, do the necessary tweaks, and when I feel that the system is perfect, I create an image of the system and burn it to dvd. before I was doing this with 3rd party sw, but now I can do it easily from backup and restore at w7.
so in case of a system failure, I restore the image, and because all my personal data is on 2nd partiton all things return to normal in minutes. I have been using this method for years and it never failed. of course if hd fails physically, that is a problem, but I am keeping backups of my data at other mediums, music at ipod, movies at dvds etc...