Did The Vostro 420 Ship With Both XP and Win 7?

No.  Dell would have only shipped the operating system you chose.  If you do indeed have a license for Windows 7 (and through Downgrade Rights had XP installed/running), then you will need to obtain Windows 7 media to do the install from DVD yourself.

No.  Dell would have only shipped the operating system you chose.  If you do indeed have a license for Windows 7 (and through Downgrade Rights had XP installed/running), then you will need to obtain Windows 7 media to do the install from DVD yourself.

Just to expand on that; Windows 7 Professional/Ultimate with downgrade rights to Windows XP Professional with Windows XP Preinstalled. Check the Code Of Authenticity.

If it says Windows 7 Professional then you are licensed to run Windows 7 Professional but if it says Windows XP Professional then you have no license to run Windows 7 and must purchase a seperate one.

I may be wrong but I think Dell were only selling XP Preinstalled using downgrade rights and no systems had direct XP licenses in June 2009.

Also check the Original System Configuration found on the Dell Support Page.

If the COA isn't faded you may follow my wiki Windows Reinstallation Guide/A Clean Install of Windows 7:

http://en.community.dell.com/support-forums/software-os/f/3524/t/19480753.aspx

A Dell Reinstallation DVD is preferable for the installation of Dell OEM Windows 7 and I would advise requesting one however there are some issues with requesting one at this time so a Digital River .iso with phone activiation to Microsoft may be used instead if the product key on the COA is completely legigible.

To request a Reinstalaltion DVD as you are a customer in the US fill out this form here:

http://support.dell.com/support/topics/global.aspx/support/dellcare/en/backupcd_form?c=us&cs=19&l=en&s=dhs&redirect=1

If you have any issues you may add your voteto my poll here:

http://support.dell.com/support/topics/global.aspx/support/dellcare/en/backupcd_form?c=us&cs=19&l=en&s=dhs&redirect=1

Also check the Original System Configuration found on the Dell Support Page.

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I couldn't find anything on the Dell Support Page, but this was one of the items listed on my original Order form:

WINDOWS XP PRO SP3 with Windows Vista Business LicenseEnglish,Desktop,Vostro

So it looks like I'll have to buy Win 7.

Yes in that case you will have to look for an upgrade license to Windows 7.