I have Win 2k and Solaris 8 on the same partition. No problem.
DON'T use the Solaris Webstart CD. Use Software disk1. This has all the tools to format the drive correctly.
You just need to make sure you install Solaris 8 FIRST, before win2K. Set a PRI-DOS partition in the Solaris install for the Win 2k partition to sit in.
After Solaris installs, boot the machine using a Win98 start disk and use FDISK to select the intended windows partition as active. DO NOT use FDISK to format the partition, or win 2K install will fail.
Then boot from the Win2K CD and install.
Then use win98 floppy/fdisk to change the partition back. Use the Solaris Boot manager to decide which OS you want. Don't try altering boot.ini, because without some extra fiddling - it can corrupt the Solaris partition.
I am still looking for a way to install Solaris 8 on a system which already has Win2K running. Will post if I find one.