Windows 7 windows 7 machine can not access win2003 machine in a workgroup

Typically this is a product of an incorrect username and password being used or cached in the credentials vault.
When prompted for the username and password make sure you use the correct format. MachineName that you are trying to accesss followed by a backslash "\" then the UserName of a user on that machine with the appropriate credential and then the password for that user on that machine, like in your case
W2k3serverName\Administrator
Password of Administrator Account on the W2k3 Server.
Additionally double check the work group name and make sure all are identical, the time, date and time zone settings make sure they are all sync'd, check and confirm that there is no duplication of NetBIOS names (machine names), IP addresses or MAC addresses, And any 3rd party security suite that is running on the problem machine (Win7) that makes it unique or different from the other machines that are not having this issue.

Funny how it's always the last thing you tried. But then if the problem is fixed why would you try anything else
Glad to hear that you were able to resolve your issue and thanks for posting back and updating your thread with the solution that worked for you.
Thanks for joining our community and we hope to continue to see you around the forum.
Regards
Randy