Yes this can be done quite easily with todays automated Linux installs (I would suggest either RedHat [www.redhat.com] or Mandrake [www.linuxmandrake.com] for beginners like yourself). Most Linux builds will automaticly install Lilo (LInux Boot Loader) and will automaticly add your dos partition to the list. So when you boot all you have to do is type either "dos" or "linux" to load the appropriate partition. This works great with Win9x->ME but gets a little more confuzzling with WinNT->2000.
Two things to note before installing:
1: Make sure you have an empty partition with at least 2+GB of space on it.
2: Make sure that your dos partition resides in the first 2GB of the drive, dos is kinda picky about this whereas most versions of Linux are not.
PS - When you install your copy of Linux I would suggest you only install a bare bones system [General OS stuff, Xwindows (GNOME, KDE, Etc.) is also good for the transition to Linux], you'll want to compile all the server software yourself for use with php, that way you can customize the system to your needs. Don't worry this is much easier than you might think. When I first learned it took me less than a day to get a working system up with very little instruction.
If you need more assistance gime a buzz, I just finished building a new server machine so it's all fresh in my mind right now.
Shaun Hawkes
helpme@phpgods.net
Victoria, B.C.
Canada