Is it possible to install unix on a system which already runs windows 98?
YES, but not on the same partition. what flavor are you planning to use? if you want to use something like linux you should try MANDRAKE 4.1 FOR WINDOWS and don't forget to make a swap partition
There is a program called VMWare that you can get a 30-day trial that lets you run a virtual machine but this requires a seperate partition also. But you run it will you are also running your windows program. We are using this at my job and it seems to work pretty good.