Whether or not the driver is digitally signed is irrelevant. Here's the link
for the AIO P924's XP driver set.
Once the driver is downloaded, double-click to begin the unpacking of the drivers into the C:\Dell\whatever folder. Once the actual setup begins, cancel it. The driver's will still be in the folder.
Now go back into the Printers and double click the AIO P924. When it screams it cannot find the drivers, direct it to the folder where the drivers were unpacked. Again, don't worry if the drivers aren't digitally signed. Click 'OK' and continue installation.
According to the user's manual, you should have no problem with printing on a network. Scanning will only let the machine save to its connected computer.