As mentioned in a recent Security Now podcast, McAfee offers free WPA client software that can be used to add WPA functionality to older operating systems. Other wifi client software may also provide, or soon provide, such functionality. For example, Cirond had promised that it would add WPA functionality to its WINC software during Q4 2005.
Wi-fi Protected Access is significantly more secure than the older WEP protocol. However, most access points can only operate in one mode (WEP or WPA). It is therefore necessary to upgrade all devices that access a particular access point to support WPA before adjusting the access point to operate using WPA.