Vista - ASP.NET is the administrator on my computer, "Access is denied"

i am curently very fustrated!!! as a new user has appeared on my computer (ASP.NET Machine Account Administrator). This user account has taken control of my computer & is the computer administrator, & i do not know the password for this account.
We have a HP Pavillion Notebook PC dv6500, using Windows Vista Home Premium 32-bit.
I have no administrator privlieges on my user account (courtney), cannot install programs, run already installed programs or do anything i want too.
Some of the error messages i have seen include: -Access is denied, FAILSAFE.BAS, System error 5, you dont have administrator rights please see system adminisrtator.......
for about 4 months now i have not been able to access my computer as an administator, CAN SOMEOME PLEASE HELP US TO BECOME ADMINISTRATOR OF OUR COMPUTER AGAIN!!!!!!!!!
we have tried alot of things already but nothing has worked, so we need all the help we can get to resolve this problem

Boot into in safe mode -if not any administrator account in safe mode should appear build-in administrator.If build-in administrator is also unavailable in safe you have to turn build-in administrator account remotely

See below advices how to do that:What do I do: I can no longer log in or elevate to an administrator account
and User Account Password - Change from Win RE

apart of above methods alsoTools for reseting password also have option to enable/disable accounts (for built-in admin also) for example Offline NT Password & Registry Editor
If you have been degraded to limited user you can also use this tool to add yourself to Administrators group