Email access from multiple computers..

Can anyone tell me how to enable users to access their email from any computer on the network? We use MS Outlook on a MS Exchange Server, computers are laptops running XP Pro. The users don’t have access to the same computers every day so can’t be set up on just one machine, and we don’t want to try and set up everyone on every machine.. Also, the laptops are mobile in vehicles, and they access the server via WLAN and Citrix.
Any suggestions woould be appreciated..