Colin's response is 100% spot on. Pre-installed systems without discs are often factory-installed in a hidden partition on the hard disk. Your documentation will possibly tell you which " f* " key to press on initial boot (it may also be displayed on the pre-windows screen on boot, just after memory test).
Be aware this will do exactly what it says - restore Vista to EXACTLY as it was when delivered. It will re-format the C: drive, thus wiping ALL the data, drivers
and updates installed since you originally got the pc.
In my experience (with ACER pc) this re-install process is relatively simple, requiring minimal user input. However, it's the subsequent updates that are the pain - you will need to download/install all the Vista updates, PLUS re-install all your applications (and their updates too) plus drivers for your peripherals. Look forward to at least 2 days work!
A tip - if not already done, partition your hard disk drive C;/into at least two (do NOT attempt to change/format the existing hidden partition else you will lose your "factory settings", i.e the entire factory installed recovery system)
When you have recovered your system, use the D: drive to store ALL your data files/drivers etc
Reason? If/when you have to "restore" again (and thus reformat/wipe the C: drive, again) it's only the C: drive that's wiped - data stored on D: is still there!