Five solutions:
1) Get a switch and two TP-cables, take your original one from the modem into the switch and then one of the new ones to the HP and another one to the mac.
2) Stupid solution: Get another network card and a crossed TP-cable, put the network card in the HP and share the connection with the mac.
3) Buy a wifi accesspoint of any brand, 802.11 b/g or even n if you want one which supports faster network speed. I guess they should work, but I doubt you can be 100% sure.
4) Get the Airport Express. This one supports 802.11 b/g which is what most people use. I don't know but probably you don't have an Internet connection faster than 54 mbps or even what you will actually get out of it so it won't matter that much that it's not top of the line. It's better than (3) because you can connect it to your stereo and printer and play music and print documents wireless.
5) Get the Airport Extreme. This one supports 802.11 n aswell which is much faster, but it also cost twice as much.
make sure u read all the posts b 4 posting we already have found that the airport express will not work because his desktop does not have a wireless card all he needs to do is get the airport extreme and or another type of wireless router but the airport extreme is best