Okay I don't know if you know anything about registry editing but here goes:
1). Click Start -> Run -> type in "Regedit.exe"
2). Now navigate to this key:
HKEY_LOCAL_MACHINE -> SOFTWARE -> Microsoft -> Windows NT -> Current Version -> Winlogon -> Special Accounts -> User List
Reply and tell me what the values are in there in mine for example there is
Administrator
James
Guest
Thanks
NOTE: Even if your running Windows XP, the Windows NT thingy will still be there.
try this, go to start/ controll panel/ adminastrative tools/ computer managment/ local users/ groups/administrators/add, then add your acount.
i dont know if this will work, but give it a go.
woz