Did you try making the usual "SHARE" on XP? and then try to see if your Apple networking can see it?
There are some thousands of web pages about that so I'll stop here.
The desktop is a Dell Dimension E310 running XP and my Macbook Pro has 10.7.5. I'm wondering if I can send files from my Macbook to the E310, preferably with an ethernet cable, like an external hard drive. I have been searching everywhere and trying everything. The closest I got was only being able to get internet to the XP and not send files. Any help would be appreciated.