The most likely answer is that you have an access control list (MAC Address filtering list) set up on your router and have not added your Mac's MAC, also called your Airport ID to the list but there are other possible causes.
Double check that you do not need to add your MAC address to the list, that you did save changes when you removed security from your router and that your PC can still connect.
The hardest part of moving from PC to Mac is learning to trust your computer to do things for you, with Microsoft's OS's you don't just trust the automated set-up wizards because even in the rare circumstance that the wizard actually works you invariably have to finish the settings manually.
With your Mac the exact opposite is true, if you try to do everything manually it'll invariably mess things up, where as using the wizards and trusting the computer to manage things on it's own reaps obvious rewards.
If Airport set-up wizard is asking for a WEP key and leaving the field blank doesn't work you can be comfortably sure that the router has a key set up on it. (BTW: Always be mindful of the fact that WEP is pointless, even 128bit.)
Check that your router has not been hijacked and that you have indeed removed the security from your router, is it possible that you have a WPA key set up on the router? Unlike WEP, WPA and especially WPA2 actually secure your network (in as much as is possible).
Another major cause of connectivity issues for new Mac users is having Norton Internet security installed, for a PC user the idea of not having full-on paranoid security installed is anathema, the idea that your Mac's inbuilt firewall alone is plenty of protection (at time of writing) Other than that ClamXav
antivirus will protect your Mac form any viruses that may appear in future.
Norton Internet security for Mac is often pushed on buyers by stores that don't know any better, to be honest Norton Internet security for Mac is an awful lot of hassle and performance loss for little or no advantage.
Using a Mac is different to using a PC but once you have got used to using a Mac you will not be able to change back, once you have learned to trust your Mac to do things for you, you will find Windows (any version) to be painfully time consuming, unstable and fiddly to use.
Post back if you are still having problems. :)