probably best place to start is here..........
Compaq Support.
before formatting, probably a good idea to write down or print out hardware devices, in device manager...mainly, the soundcard, modem, video and such.
According to Compaq...that board is an 815e chipset....you can probably download latest INF drivers from Intel.
Compaq lists drivers for the souncard, which you can get....but not the modem, that might be your only headache...it lists 56K ITU v.90 Modem, but no drivers for Win98 drivers are listed.
But, there is a driver for PC-TEL Platinum Series V.90/V.92 Modem Driver for Windows XP ....so before you begin, make sure your ready, no going back.
Did that machine come loaded with XP or Win98??? If it came loaded with XP, i don't understand why you wanna go back to 98 for? XP is far better....just curious.