HDD light blinks every second.

I just installed Windows 7 Ultimate to my new PC (was XP before) and I noticed that the HDD light blinks every second even if the PC's idle. Is that a normal behavior for Windows 7?

In the early days, yes.

Windows 7 uses Superfetch to quickly learn which are your favourite programs and this task plus indexing means there will be a lot of hard drive chatter, which of course is more noticeable when your computer is idling.

Give it a few days and things will quieten down.