WPA is the Wi-fi alliances application of the pre-standard 802.11i. It was designed as a backwards compatible software enhancement of WEP (WKIP/RC4 vs WEP/RC4). As such, it does not require any dedicated hardware as WPA2 (CCMP/AES) does, however, the capability to support WPA is written in the firmware of your network card, not your OS.
If a network card doesn't specify support for WPA in your current firmware version, then you are unable to use WPA.
I agree initially however you do also have operating system issues to consider if it isn't up to date. Windows XP SP1 did NOT support WPA properly at all, some of the older versions of Linux do not support WPA etc etc. If you have a reasonably up to date Operating System (Win XP SP2/Vista, newer versions of linux using a 2.6 kernel or higher etc) then you'll need to verify your hardware as Iainw said.