How can i restore an old Vista OS without the backup disk?

Your computer manufacturer is probably your best bet for getting the restore disc(s) you need. But have you looked on your computer to see if there's a way for you to recreate it, or just do the restore? I think my 4 year old HP desktop had something under Start -> HP ->... for doing that. Also you should check your user guide which again should be on your computer. If not, you should be able to download it from your manufacturer's web site. Finally I'd suggest you boot to the Advanced Boot Menu (by hitting F8 about once a second when powering up) to see if there's something there to restore to factory settings.
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When you've gotten your system back like you want it, I urge you to get yourself an external hard drive and a good 3rd party backup program. You can set it up to do everything automagically at the time and frequency of your choice. 1 TB external hard drives are about $70 these days and a really good FREE backup program is the Easeus todo backup free which you can download from
http://www.todo-backup.com/products/home/free-backup-software.htm . That can save you a lot of time and frustration the next time something like this happens. Sooner or later it happens to all computers for one reason or another.

Good luck.

Dell has a download system noted at link.

http://forums.cnet.com/7726-13974_102-5325452.html?tag=posts;msg5325452

Please don't forget to add make/model numbers so folk can give you the best answers.
Bob