This can be done by using the Auto Logon procedure.
So when your client connected your viewer with SC, asking them to enable Auto Logon too -- you need to ask them to do so because this procedure need him/her to input the password, or unless you use an A/C that you know the password. After enable this feature the computer will logon to that client's A/C automatically when reboot.
Upon complete your job, just disable the Auto Logon then the pc will resume as before when boot up.
See how to enable Auto Logon at:
http://www.microsoft.com/windowsxp/usin ... logon.mspx