Okay. Unfortunately we can't transfer hard drive space from the Windows side without a significant amount of work and the install disk. The hard drive filling up can also cause damage to the Mac operating system that we cannot repair without the install disk, but there is one more thing we can try first.
You'll need your friend's Mac and a FireWire 6-pin to 6-pin cable (you can find one at Best Buy). The ends will look like this:
Once you have those,
1) Connect the Firewire cable between the Macs
2) Start up your friend's Mac
3) Start up your Mac while holding down the T key
(More information: http://guides.macrumors.com/Target_Disk_Mode
This will turn your computer into an external hard drive that you should be able to see on your friend's Mac. Once you're there, you need to copy off and/or delete files from the hard drive on your Mac to free up some space. You should have at least 1 gigabyte free on the drive for the operating system to have a comfortable breathing room. Make sure to empty the trash when you're done, and try to stay away from the system files when deleting things. If you don't know what a file is for, don't delete it.
Once you're done and have free space, drag your hard drive into the trash can on your friend's Mac to unmount it, shut it down, and try rebooting again while holding down the shift key. If that doesn't work, try holding down the option key to select Mac OS X.
Also, any troubleshooting steps beyond this are going to require an install disk. If you know someone with a similar model of computer to yours then you can use their install disk, otherwise you'll need to buy a new one.
Let me know how this works for you and I'll help you as much as I can. :-) Best,