It is possible to share files between a Mac and a PC and it is much easier than it was just a few years ago. This is the most concise and easy to follow instructions that I have found so far. The operating systems in use here appear to be Windows XP and Mac OS X (10). Follow the instructions and you should be up and sharing in just a few minutes.
Keep in mind that if you are using a domain connected PC that you will need to supply your full username (not just a simple username) with the domain in the Mac window that request for authentication for your Windows shared folder. You could alternatively create a local user account for authentication purposes. Your full username should be domain\yourusername or yourusername@yourdomain.