Helpdesk Remote Access Software.

I am looking to start a remote help desk for my clients [As in Customers not Network] but have problems in locating the required software. I need to access Windows systems from 98 to XP and be able to take full control of the system to allow Virus removal, registry editing, program installation etc. etc. I don’t wish to spend large ammounts on licences at this time.
Any ideas on what is available and good to use.
Regards
Dave.