[HELP] User Account

Hello peeps.

I forgot the administrator password & log-on using Guest account currently.
All the program run needed to passby UAC (User Account Control) permisson.
How could i reset it without permisson to continue?

Do i need to format the desktop?
If do so, please tell me how to reformat my computer without disc installation.

Window Vista Home Basic
Service Pack 2

System
Manufacturer: Dell
Model: Vostro VOSTRO_200
System type: 32-bit OS

Hope anyone can reply me soon!
Thanks.

Best Regards.