Indexing Service
When Indexing Service is enabled, the files on your computer are indexed and maintained while your computer is idle so you can perform faster searches.
I am 99.9% sure, this is one of the reasons why the hd light is flashing. I created a dual boot of two Windows XP installations today, and set the indexing service enabled in one and disabled in the other. As expected, the hd light started flashing after a while when the computer was idle in the XP system where indexing service was enabled.
cccept got his problem solved after turning off the CD autorun, but mine didn't; neither didn't for freezer121. To most of the others, it may be other disk maintenance utitiles running while idle, and you forgot. In my case it is 100% it is because of the indexing service.