I had the same problem with a Pentium 1-60. Your 486 dates to around 1990-1992. Hardware was changing every couple months. PCIBus was just beginning and was not in it's 1st generation configuration. There are no PCI slots in your computer and the CDROM does not plug into the motherboard, it connects to an ISA card. ISA is the bus (card slots) used at the time. You need the manufacturer's startup disks. Because it's Compaq (now owned by HP), you may have a slim chance of locating these.
Even if you can get Win98 to load and run, it will be extremely slow. Minimum hardware requirements are: http://support.microsoft.com/default.aspx?scid=kb;en-us;182751