I was reading a Wikipedia article about IIS and I found a comment about limiting of connections under XP and Vista. I presume that there's no limitations under Windows Server, and is just to get people to pay for servers?
IIS, as far as I know (I.E. according to the page on the MS website), contains no limitations under Windows Server. The reason they do this is simple, they want you to pay to use it as a server.
They allow a basic version of IIS in XP/Vista/7 for development locally, it contains all the features it's just limited. It's not intended to be used in a production environment because the OS isn't built for that. I guess it's similar to WAMP/Apache, WAMP is great for local development but probably shouldn't be used in production.
If all you're doing is running a little hobby site that might peak 100 hits a day, IIS XP would probably be OK, and if you just want to play around with the technology it's great for that too, but Windows Server is built to be a server OS (I.E. on 24/7). While XP/Vista/7 may be capable of 24/7 workloads they weren't really built to do so. If you're a student you can get rediculous discounts on Microsoft software (providing you have a valid student ID), and if you're a startup you can apply to BizSpark to get discounts on the software (I think they might even do it for free in some cases). And even if nether of those apply to you there is a 90 day trial of Server avaliable for download from the MS website which gives you plenty of time to play around with stuff before deciding to buy.