Its easy. First and foremost save all your documents, pics, etc. to a flash drive or external hard drive, someone else's PC, whatever.
Then put your Vista Operating system disc in the DVD-ROM.
Restart the PC, most likely you will see something to the effect of "press any key to boot from CD-ROM."
Press the any key.
Vista will give you two options, repair or install.
Do a clean install.
Only the Vista OS will be installed, no dell trial period spam.
Vista will re-name the old install as windows.old. You can delete this after you confirm all is well.
Again, get everything important off the PC first. You may even choose to use "Windows Easy Transfer" to migrate your stuff somewhere else. Make sure you use the Vista disc that came with your PC, not the Dell recovery disc. Since this is your first go at it, I would recommend you have another PC lined up to browse the web with in case you get stuck or need to DL drivers or something.