just get a Mac, you won’t regret it (even a Mac mini if you cannot afford more than $600)
How about Linux/Ubuntu? Is Mac much better than Linux too?
You could start with a VM running Ubuntu to get started with Rails development. A Mac would be far superior, but Ubuntu will get you started. You can do Rails development on Windows, but you may have trouble with certain gems.
J
From a purely RoR developer perspective Linux is good. Which you would
expect since most RoR applications are deployed to Linux servers - or
are you deploying to Windows servers. Its just that Macs are nicer 
As to why your Windows system is slow for developing could be that you have a crappy system. Windows doesn't help but even Linux will struggle with an under specd machine. Is your hardware up to snuff?
Given enough hardware I'm sure that a Windows development machine will fly but there is an issue of getting gems to work. We had a developer here who started out with Windows and had no end of trouble getting some gems to work. He finally jacked it in and got a Mac and stopped swearing so much.
My preferences are:
1) Mac = In heaven
2) Linux = No problem, I can work with this
3) Windows = If this was a job requirement I would not apply 