I would like to offer a tutorial link that you should read from M$http://support.microsoft.com/kb/304040
Many of the principles here are handled similarly in 2000 except the User interface may look slightly different.
Once you have wi-fi networking up and running, you can share files between both computer. The default that occurs when you attempt to share files, is that XP will start a guided wizard which doesn't necessarily work out of the box. Also, the wizard may not be the best option for you. check out the article and then get back to us. Further, we may need to troubleshoot firewall issues as many of these block file and printer sharing out of the box.
Also, being the advocate for security that I am, type cacls on google or wikipedia.org cacls is a command line tool for setting file and folder permissions which we may want to do. In addition, XP and 2000 offer a dialogue box to change
file permissions which is confusing at time but easier to navigate than cacls.
Tell me what you have for Firewall on both pc's and what type of wireless encryption you're using.
Finally, explain where the files are that you wish to share.