For last few years, I've been saving all my install files on an external hard drive. Doing this has really helped me whenever I have to re-install my operating system. I've had to re-install Windows XP twice, not counting the time that the Lenovo repair depot needlessly wiped my laptop back to factory state.
Starting over from scratch, it's handy to have all my install files on-hand without having to download them again or even remember what I had on my computer . More importantly, if you buy software that's only available online, you can only download it once. If your computer crashes and you have to re-install your operating system, that is not a "valid excuse" for getting another copy of the software. Believe me, I tried.
If the install files require license codes, make sure you save those, too. And don't forget to backup the install files!
 Alternatively, you could run Belarc Advisor on your Windows machine to get a list of all installed programs on your computer. Belarc Advisor is also handy for getting hardware model/ID numbers.