Laptop IP address changing on its own

[Deuxdad]

My wife, who expects her new HP laptop to work like a toaster (no thinking involved) is getting increasingly frustrated because she frequently can't print to the desktop printer. The setup is:

a)Desktop computer (WinXP-pro)hard-wired to Netgear router.
b)Using free Zonealarm for desktop firewall.
c)LaserJet printer hard-wired to desktop.

a)Laptop computer (HP)(Win Vista Home Premium) connected wirelessly to Netgear router.
b)Laptop also using free Zonealarm for firewall.

As long as Zonealarm on the desktop has the laptop IP address correct, there is no problem and she can print to the LaserJet printer. However, all too frequently the laptop's IP address changes by itself requiring me to change the laptop's IP address in Zonealarm. Once done, she can again print to the LaserJet printer.

Why would her laptop's IP address change by itself and is there any way around this nagging (no pun intended) problem?

TIA,
Deuxdad