How do I Print Address Mailing Labels from Avery 300-Easy Peel White Mailing Labels, on Windows XP,CP?

You'll need the actual Avery code for this, or at least the correct measurements.
Open up Microsoft Word. Go to 'Tools' and find 'Letters and Mailings' in the list. Then off of that is 'Envelopes and Labels'. Click on that one and a box will pop up. If you click on 'Options'. From here you can find the correct label settings by the code which will be on the Avery pack. If you haven't got the code, you can scroll through the lists and highlight options to see the measurements but this will take you a while.
Once you chosen your labels, click on ok. Now you can enter your address in the box and print a whole page of the same address or click on 'New Document' and that will give you a page set out correctly for that label sheet.