The manufacturer installs their OEM version of the Operating System into the C: partition. A factory re-install of the C: partition is accomplished via the Recovery application. The recovery process puts the machine back to 'Day One/ Out of the Box'. It is stored on the D: partition. This partition must be protected. Do Not set 'restore points' to write to this partition. Do Not use this partition for anything (pictures, games, etc.)
1) You do not state how you accomplished turning it back to the factory settings. There are only 2 ways to do this:
a) From a set of Recovery DVD's, either furnished or created by you. Alas, you said 'but I don't know much about computers'. You really need these DVD's because if you hard drive became 'Dead/Fried/Useless', you would have means to install the C: and D: onto a new hard drive.
b) From the D: Recovery partition. The most logical, given your stated ability, was that you entered it before start-up via an 'f' key.
But you said something that contradicts : 'I have made backups of all my important files from the D: onto an external harddrive'. That suggests the D: partition ismuch larger than 10 GB. (I am gauging that by the space required for Vista Ultimate Recovery).
Formatteren met Windows 10 Om een usb-stick met FAT te formatteren ga je als volgt te werk. Vervang nu onder Partitie de optie Huidig door 1 en klik op Opties.
Windows Vista Partitie Formatterencer
2) That simple statement implies that your D: partition was compromised, or never setup as a Recovery partition.
a) Did you purchase this computer NEW? It is unlikely that a manufacturer would not install the D: Recovery application. Although my HP8100CTO has a D; Recovery partition, I was allowed tocreate a separate set of DVD's for that purpose. (see 1,a).
b) Just how large in GB's is your C: and D: partitions?
Windows Vista Partitie Formatterent
I do not feel comfortable suggesting you format the D: partition, without having some answers as to what is in this partition.
BTW: I am not affiliated with MS. I am just a technical user enjoying these forums, and adding to my knowledge.