Hi chippy,
The answer to your question is Yes and for this you will need to set up a network connection between the two and allow the file to be shared.

Now then, How to do it is the next to be answered and as networking is not my forte I'll mark this thread as Open, this should help to get a response from those who know.

Thanks for your patience

Hello Chippy,

Having a bit more knowledge about your PCs and your home network would be most useful. Please use this document to post information about your primary PC. Following these steps places in the information as an icon available on each and every post you make going forward. This will stop our techs from repeatedly requesting the same information.

Most important in order to get the correct information to you is knowing the Operating Systems of both PCs.

they are both XP

Chippy,

Microsoft makes the following documents available to assist you in your home network and subsequent wireless networking configurations.

Set up your computer for home networking

and

Set up a wireless network

ok thank you i've set it up now and it seems to be working fine. i can access my music file from upstairs now.
couple of questions also though:
1. Can i make this file private so that only MY pc upsairs can access it and not my sisters laptop?
2. Or if not, can i put a password on the file to access it?
3. Does the computer downstairs have to be on for me to access the file? Becasue it doesn't seem to be working now since i shut it off.

1. I haven't done this on a home network only at work, but yes, you can assign rights that will control who can access
2. that is an alternate - also likely easier
3. absolutely, the computer hosting the files MUST BE ON to access the files

Let me know what you would like information on. I'm sure the documentation exists at Microsoft. Just takes a bit of searching.