My brother has stopped using his old Windows 98 computer, so now it is just collecting dust. I have no need for a 98 machine, since I use XP, but if it is possible to install Linux on an old computer that used to use 98, then can someone tell me how. I will format the hard drive and install linux.
If I can use Linux on it, what is the best distribution to use with an old computer.

How old?
Post the hardware specs.
Slackware is a good choice with older PCs.

Check out http://www.linuxfromscratch.org - you can find good advice on building a system from scratch there.

Building from scratch allows you better/full control of installed packages/programs and you can avoid too much bloat that distros can encumber you with, plus you'll learn heaps if you follow the steps available in the online/offline book.

Linux From Scratch is indeed a powerful tool, however, I would not recommend it to a newbie who hasn't worked with Linux previously.

To second what michaelk stated, if you specify the specs about your machine (CPU, RAM, hard drive, motherboard, etc) it would be helpful. That being said, IMO the critical component with older PC's is RAM - the more the better. For comparison, one PC I've got is a Dell Dimension with a P3 @600Mz, 256Mg RAM, a 40G drive, CD-RW drive, using onboard sound and video, and it's running Slackware just fine. Good luck with it -- J.W.