Sounds to me like you are getting BSOD (Blue-Screen-Of-Doom) crashes that aren't being shown because you have the "Automatically Reboot on System Failure" option turned on. You should turn this off so that you can see the actual system error message. Then you can easily google search the STOP code and find out (at least somewhat) what's happening.
To do this, right click on My Computer and choose Properties. Switch to the "Advanced" tab and click the "Startup and Recovery" button. Uncheck the "Automatically Reboot" option under System Failure. This will let you see the actual BSOD. Microsoft chose to hide them by default in Windows XP for some reason (hmm...maybe because people make fun of them all the time).
My guess? Probably an IRQ_NOT_LESS_OR_EQUAL error caused by your USB root hub drivers or your USB drive drivers. If this is the case, I would look to Intel's web site (and the web site for the drives) for driver updates. I'm not familiar with the Intel 875P chipset (I have switched to AMD processors) but I know they used to regularly release chipset driver updates. ASUS doesn't always post the most recent on their web site, so I'd probably go directly to Intel.
Also, make sure you have the latest and greatest BIOS on your motherboard and USB drives. I checked for you quickly and the most recent ASUS BIOS for your motherboard came out on October 29th, 2004 so unless you've updated since then you probably don't have the newest one. Even your internal CD-Drives may (probably) have Firmware updates, so check for them. You can get the ASUS BIOS here:
http://www.asus.com.tw/support/download/item.aspx?ModelName=P4C800-E%20Deluxe&Type=LatestHope this helps! In the end, it could be that what you are trying to do is just to intensive for the USB bus, and you may need to add separate USB2 interface cards to the system to use that many high-bandwidth devices at once. Just because they give you 8 USB2.0 ports doesn't mean that the motherboard can supply the full USB2.0 bandwith to 8 devices simultaneously (in fact this is almost certainly NOT the case).