Right 2 ways, wired or wireless. Both ways would mean that if you wanted to use the laptop for internet then the desktop pc would have to be on.
1. WIRED - Get a PCI Network Interface Card (NIC) for your desktop unless you desktop has an ethernet port at the back. Get a PMCIA Ethernet Card (unless laptop has an ethernet port) and a length of ethernet wire to go between them (up to 35m i think, not sure on max). Just install the cards, windows should recognise both right away very easy to install, then run ADD NEW NETWORK on XP machine when thats down select the create a disk (comes up part of option) and then run that disk on the other computer (win98 one)
D-Link 10/100 PCI Card 7.04 click here
D-Link 10/100 PMCIA Card 22.25 click here
Wire: get from local small pc shop works out roughly pound/metre and they can make it upto any lenght i find better value than online.
2. WIRELESS - Get a PCI wireless card i they start from 25. Get a compatible PMCIA card (same wireless freq.) and install and then run the network setup program on XP as mentioned above and create disk and install on 98 PC.
Belkin PCI 802.11g card 31.49 click here
Belkin PMCIA 54g card 24.49 click here
802.11g & 54g same things, i only chose these as well known company and offer fast acess speeds.
An Alternative is to get a wireless/wired router connect your modem to that and so either computer could access the net without other one being on but not sure how exactly works with connecting modem...