Simples .. and I've just done it for a friends laptop.
You can use any copy of Vista you like, as long as it has the same "version" as yours ... Vista Home basic or Vista Home Premium, just borrow one from a friend. As long as you use your own serial number, which should be on a sticker attached to the laptop, and also says which version you have (had) you are acting totally legaly.
So, get hold of a disc, set the system to boot from the CD, reformat the drive, install Vista, and you have a clean install, but may have a few missing drivers.
Using another system that is connected to t'internet, down load the drivers - usualy available from the makers site .. even if archived - copy to a memory stick and transfer to the laptop.
Once the laptop is running and has internet access you can , quite legally, and with no problems, "activate" it then do all the windows updates.
Takes a while but is very simple to do.
If you can't get hold of a Vista disk, I can loan you one .. just need to know which version.