On the vista machine, go to control panel>network and sharing center and click the little grey dot next to file sharing and click turn on file sharing. On the XP machine there should be a folder called shared documents in my computer. If not, right click my documents and select properties and there should be a tab with file sharing or something similar on it. Tick the box to allow file sharing and ok any warnings. Now you cann drag and drop files into that folder. On vista it's under computer>network (on the LHS list)>computer name>shareddocs. Under XP it's under my computer>shared documents. Make sure both computers are in the same workgroup. To change this in XP, right click my computer, click properties and it is under computer or one of the tabs, i can't remember exactly, but its the tab with the computer name and description. On vista right click computer and select properties, under computer name, domain, etc click change settings, then the option to change the workgroup name is under the computer name tab.
Sorry to be long winded, hope it helps!