Converting Mountain to Commuter

[Trek420]

I commute and sometimes do road rides (even slower than I normally go) on a cheap lil' 10 year old GT Outpost trail.

Steel frame and fork, no suspension, nothin' fancy, cheap cheap cheap bike. It had slicks from the start. Last year using a gift certificate from family for my 50th I got new wheels: Mavic Cross Ride.

Whoo hoo! They cost more than the bike itself but dayuhm I love this bike. I may even re-paint it I like it so much. Best upgrade ever.

[SalsaMTB]

Ditto what everyone else said. But, just to get you riding, I would recommend the below:

Get a tail light, front light and reflective tape (you never know when you'll get stuck at work late)
Backpack
Basic tool kit to repair a flat or chain
Bike lock (if needed)

All other things are nice to have if you can afford it. If you're looking for the necessities, I wouldn't ride without the above.