This is a difficult one to do. First, it is easy to edit the boot.ini to point to the old installation on the second drive.
http://support.microsoft.com/kb/289022
http://www.pcworld.com/article/14161...g_systems.html
http://vlaurie.com/computers2/Articles/bootini.htm
However, you might run into drive letter assignment issues that could hose both systems, so make sure you run backups. I haven't messed with a setup like that in about 9 years, so I don't know for sure what other issues you might have. But it is worth a try!
The other option is to get an Ultimate Boot CD, partition the drive on the newer machine into two partitions, one with the new stuff and one with the old stuff. Copy everything from the old hard drive onto the new hard drive, and then boot to that. Or just throw GRUB or LILO on there and be done with it. But that you'd be on your own with. Or you can contact me and I can do it for you, but it would probably be cheaper to leave it as is and reinstall the applications from the old computer onto the new PC and then copy the data over.
http://technet.microsoft.com/en-us/l.../bb457074.aspx
Good luck!