You are in a SQL 2005 forum.
For SQL 2000 you have a couple of options that I can think of. It will depend heavily on what the web service does.
1) Create a wrapper for the web service that you can run from the command line and use xp_CmdShell.
2) Create a com object that calls the web service and use sp_OACreate.
3) Create a queue table and either use Notification Services or create your own polling service to check the queue and then call the web service.