You're asking two questions there, which I suggest we take one at a time.
First, accessing files between XP and Windows 7 is certainly do-able, I do this at home. Just want to check though before we get into specifics how your two machines are connected together. I'm presuming you have some sort of internet router and both your PCs connect to that either via wireless or a physical cable. Am I correct?
Here's the abridged version then:
1) Make sure both your computers are in the same workgroup
2) On your XP machine right-click on the folder you want to share. Choose Sharing and Security from the menu. In the box click to share the folder and give the share a simple name. Click OK
3) Now on your Win 7 PC go to Start -> Run and type in \\name-of-XP-PC\share-name and OK that
4) If your two machines can see eachother OK it should come up with a username/password challenge. Supply a username and password that you use on the XP PC
5) If all is well you should be able to browse your files
Let me know how you get on with that or whether I haven't given enough detail
Set one up; you'll need one to access the share even if you don't use it for anything else. You can do it by right-clicking My Computer then selecting Manage then opening the users part and adding one in there. For initial testing I'd make it a member of the Administrators group once you've created it to make sure it has the rights to access your shared folder. You can always restrict it later