You can't get a driver's license without registering, your can't drive a car without registering the vehicle, and you can't vote without registering. Deal with it. If you didn't register to vote you could go up to multiple polling places and continue to vote multiple times. That is not how it is done in the U.S.
Also every voter has a special ballot based on your home address. Different members of the local Board of Supervisors, the state House of Delegates & Senators, and the Federal House of Representatives all have different district boundaries for the citizens they represent. You must register to vote so you receive the correct ballot with the exact candidate names that want to represent you in local, state, and federal elections. Got it?
If you don't register to vote you will never be a 'first time voter.'
Since you want to "just walk in" get off your butt from your computer and go register in person in Room 323 at 12000 Government Center Parkway, Fairfax, VA. This is for Fairfax County citizens only.
Detailed info:
"Normal, in person business hours are Monday, Tuesday, Wednesday and Friday 8:00 a.m. - 4:30 p.m., and Thursday, 8:00 a.m - 7:00 p.m. Please note that office hours are extended for statutory deadlines.
Fairfax County Office of Elections
12000 Government Center Parkway, Suite 323
Fairfax, Virginia 22035-0081
703-222-0776 phone
703-324-2205 fax
703-324-4700 for 24-hour recorded information
http://www.fairfaxcounty.gov/elections/vregis.htm