Win and Lin an surely co-exist on a hard disk. Afterall what are boot-loaders for

My 80GB Hard disk has Win98SE with PCQ Linux 2004 and Knoppix. I plan to install Debian Sarge on it soon
Well, but for this to work, you need to partition you hard disks properly. There have been many threads here on this topic earlier. Take a search here. You can find the search link at the top of the pages here.
Anyway, keep in mind, that you must install Windows first and then Linux. Otherwise Windows will over-write the Master Boot Record and your linux will seem to have been lost. It can surely be recovered with very little effort, but why take that trouble? Remember to install Windows first