PLZ Help w.v.h.p with command prompt /ICACLS

Hello,
Im having trouble with my compac presario windows vista hp ...I can log on as my guest user I get to command prompt and I want to grant full or change the access rights of the user, or remove all occourrences of denied rights since I can't remember my administrator password I might then be able to change password without getting denied access..or obtain full access rights..I can see the info in ICACLS but cannot figure out the file/directory/syntax can someone please give me a step by step process starting with C:\Users\Guest>..........................
I tried but I get "no mapping between account names and security IDs"
I typed icacls file /grant \Users\Guest:f
" " " (f)

THANX