i want to format my laptop but i don't have a backup disk so if i install a fake windows vista copy

but the same edition (home) as the one i have and write the (windows vista) serial no# i have on the back of my laptop i install it should have a

windows vista genuine on my laptop again right???
Basically, yes. However, you need to install a trial of the EXACT build first so your serial number could apply. Otherwise, it won't even accept your serial. What is your laptop, may we ask? Full model type please. Thank you.
im not sure what you mean by full model type but it is a hp dv5 1135ee
i hope that answers ur question

also question how can i know the exact build of my windows
Got to start, run, regedit, and scroll to the following key
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion

Also, the DV5 series come with a recovery disc maker. Did you not make a recovery disc? If not, do that. You only get to use that once, but you can make copies off the final disc.