Optiplex GX-280 and "sleep" mode

I have a Dell GX-280 that my daughter uses for a video server. It
connects to my LAN via Netgear wireless NIC. She accesses it from
anywhere in the house and sometimes has to transfer very large files
back and forth from her laptop. GX-280 goes into sllep/hybernate mode
regardless of Windows XP SP3 power settings. Power management cannot be
disabled in the BIOS (A08, the latest)--you have only the choice between
S1 and S3. This is a problem with Dell machines in general--I have seen
many models of the Optiplex systems with the issue. We can force the
machine to remain active by playing an mpeg file in an endless loop, but
despite the nominal 2.8 gHz rating the machine is pokey as crap and I
hate wasting the CPU (yes, the machine is scanned and is malware
free--it's just a dog).

Has anyone ever found a way to actually disable power management?

Thanks,
Phil Gilmer