Cmbrando,
First, there is nothing that says you have to use Compaq parts in this system if you have problems. If your 40 gig hdd dies, nothing is stopping you from going out and purchasing the first hard drive you find and dumping it in your system and reloading the software. Or, so far as that is concerned, there is nothing stopping you from adding a hard drive you need if the 40 gig is too small. However, there is one thing I'd recommend you check. Do you have a single Quick Recovery cd or a system restore cd set? the QR cd is just one cd and uses the System Save partition on your hard drive to restore, the other is 2 or more cds and will put the system save partition on your hard drive before recovering the information from it to your C: partition.
I do not know about XP identifiers. I do know that XP does want you to re-register every time you change the hardware configuration. Other than that, I haven't heard anything that is actually verifiable. Verification is the key to any "known issue" that people tell you about with Compaq or Windows. I've worked on Compaq for about 5 years now, both on a retail end and on a tech support end and have seen just about every 'known issue' be proven wrong. The only upgrade tip I know about is making sure you set jumpers to cable select if you do add drives.
If you need to call Compaq and get the System Restore cd set, the phone number for Compaq Software Support is 1-800-952-7689.
Good Luck and Have Fun.
hg