|First of all, the Dell Latitude D620 is not a bad laptop/notebook at all.|
I have had/installed/sold several of them and I am still using its successor myself (D630) on a daily basis.
If your D630 has 1024Mb or more memory it is perfectly able to run Vista smoothly.
However you have to know how to do a good and clean installation of it.
Best is to use the Original Vista recovery DVD from Dell (I prefer to use the Brown DVD with Dell Vista Business including SP2) since it will activate Windows Vista automatically.
If you don't have a Dell recovery DVD at hand you can use any DVD with Vista and use the product key on the bottom of the laptop for activation.
After doing a clean install of Vista, chances are that you will not have a working NIC (Network Interface Card) of Wifi adapter.
For that you will have to install the drivers, either from another Dell DVD which has Drivers & Utilities on it, or by downloading the appropriate drivers from the Dell website.
If this is all too complicated for you it is best to ask someone with knowledge to help you or buy another working computer.....