For the 1999, you reset by; Putting the ignition key into position I, push in and hold the odometer reset button (use your left hand), move the ignition key to position II, when the service light begins to blink,let go of the odometer reset button. (from the VolvoSpeed website)
--
Current owner of a '04 V70 2.5T. Previous owner of a '94 850 Turbo Wagon

You need to go to a service place only for the 1998. My place charges $5. The light goes out after 2 minutes anyhow.
On the 99, there is a procedure using the odometer reset. Loook around on this site, you will find the procedure.

This may vary by year and there are several threads on this if you do a search.

But for 98 model S70's, resetting the service light requires a special gizmo than costs about $150 yo purchase or you can take it to any certified repair/maintenance center and they will do it for you (sometimes for a small charge, or included with routine maintenance).

I have tried every other home remedy mentioned just to see if there was another way to reset the light. No luck yet. The only home remendy I have not tried, is disconnecting the battery for a while to reset the computer. This will also reset the radio (no big deal, just need the code). Please post if you find any other solutions than a service center.

Chris