The problem is likely your ABS Control Module. This will cause the ABS, TRAC and the Check Engine light to illuminate (p1550) The ABS Control Modules fail regularly on most Volvo's from 1996 on. There are businesses who will rebuild your control unit (that should tell you how common this is) and you can buy some units that are refurbished. Or you can get a new one.
The other code (p1171) is a fuel trim code. This can be a vacuum leak or a MAF Sensor, but the code may have also been in the memory for a long time too. A good scanned will tell you how long ago the codes were set, and if the p1171 was set a long time ago I wouldn't worry about it, I would address the ABS problem and reset the codes.
The 2 codes you have are from the engine control module, you will need to get the codes from the ABS as well, when you do, post them and I can give some more info about the ABS problem.
http://repairpal.com/volvo-s70-1999/common_problems