Buy an OEM copy of Vista Ultimate for $200. You can transfer the OS to your new computer purchases. Don't share your serial number or let your buddy install on his computer and you should be alright with activation. When it comes times to sell your computer, use the restore cd to put the OS that came with the computer back on.
If you reject the EULA, I have read that you can get a rebate from Dell, HP, or other manufacturers for the VLA copy they sold to you. It could be $50-$100 so you could recoup your investment of the OEM disk in the future.
It is not worth the hassle to download online since Microsoft will block you from future updates. They scour the bittorrent sites looking for pirated copies and blacklist those. An OS purchase is good for 4 or 5 years versus one year for most other software.