Windows likes to think that it should be the ONLY OS you're running on a machine, so it overwrites your MBR. Did you make an emergency linux boot disk when you installed RH like a good sysadmin? Use that to get back into your linux partition, and then just reconfigure LILO for your new Windows partition (you may not even have to change anything...Window's keeps a pretty standard install) and then run LILO again. If you didn't make a boot disk when you installed RH, you may still be able to create one if you know the partition information. For help on creating the boot disk, take a look at:

http://www.tldp.org/HOWTO/Bootdisk-HOWTO/index.html

Good luck!