Drive form factor will be critical. If you have old IDE drives and the new mobo has SATA connections, you will need to bit copy (Ghost, Drive Image, etc....) your boot IDE drive to a SATA drive. That is going to take some hardware and software you may not be familiar with. I know it can be done because I have done it several times, but it depends on your familiarity and computer experience level.

Your easiest option is to order a computer with Windows 7. If you enjoy the challenge, go ahead with parts. You CAN still buy motherboards with IDE data bus but they are indeed going away as Bob the Jazzmammal said.