Well don't be too concerned about it never getting a "rest". The expected life of most drives is 5 years or more. There's also actually an expected number of times the drive can stop and then spin back up (50,000 times for a Seagate 7200.7).
How recently did you upgrade? WindowsXP has an automatic "optimization" routine for the file system. It essentially starts to defrag the drive when the system is idle for a certain period of time. It only does a partial defrag though, just moving data around for the best access based on recent usage logs (XP logs every file that is used when you run a program). This can take a while when you first start using the machine, until it's had a chance to see the "normal" usage patterns and get everything in place; then it will happen every once in a while.
Also, XP has the Indexing Service, which goes through every file on the drive looking for text strings, to make searching faster and more thorough when you search for text contents of a file rather than just a file name. Depending on how many files you have, this can take a very long time, and it also gets done on a regular basis.
Both of those are however pretty continuous usage for a period, then it stops mostly.
You may need to check the power settings in XP's Display Settings, under the screensaver tab. Make sure it's set to power down the hard drive.
This may also be due to activity coming from your cable modem. The OS may be having to process traffic coming from the Net, resulting in it needing to check the drive once in a while.