Vista - Reinstalling Windows

I have a Dell Inspiron 1420 laptop with Vista Home Premium x64 that I'll be selling soon and I need to wipe everything off the computer before I pass it along to a total stranger. The problem is I no longer have the recovery or OS cds or anything that came with the computer. What do I need to go about doing in order to return the computer to the way it was when I bought it--clean?

Thanks.

As I see it, you have two very different issues:
1) Wiping information from the hard drive
2) Restoring the original Dell installation

In terms of the first, I don't know what you have access to in terms of disk utilities, but Dban ( a Linux utility) is free and provides an easy mechanism for "wiping" your drive of all data. See the following link for info: Darik's Boot And Nuke | Hard Drive Disk Wipe and Data Clearing

As to the second, your best bet would probably be to contact Dell and see what they are willing to provide in terms of restore disks.