This is quite a complicated and challenging thing you are trying to do.
You will need to learn about network setups, DHCP, PXE, BINL, TFTP, RIS, and other network boot technologies, simply as a background to getting this started. Of course, the machine with the hard drive you want to share has to be a server, because it has to run all of the above software.
If you have everything (DHCP, PXE, etc as above) running properly, then you can start looking into the solution you want to use to allow the diskless computer access to the disks on your other machine.
If you are ok with running Windows98 on the diskless computer (doubtful), you could use 3COM Virtual LAN drive. It does exactly what you are asking.
If you want to run WinXP or Vista on the diskless computer (more likely) then you need to begin learning about iSCSI initiators and targets. The computer with the disk will have to have an iSCSI target software installed on it, configured to export the disk you are sharing. Then you need to discover how to boot WinXP or Vista over iSCSI. There are documents about how to do this on the internet. Most of them involve purchasing a $500 iSCSI adapter, which is basically a very powerful network adapter with a smart BIOS in it that connects to an iSCSI target and boots off a remote iSCSI disk.
As an extra bonus, if you successfully make it this far, and have such a solution working, you will have become an intermediate network expert along the way.