ABS and Battery/Electrical warning lights

[xroadie]

I was driving down the road today and at a stop light, I noticed that both the ABS and Battery warning light were on. When I pulled away they faded and went out at the same time. Then a few blocks later, as I was approaching home, they both came on again. I let the car sit a for a while and when I start it up they both come on and stay on! Why would both of them be coming on at the same time?

It is a 2000 Boxster with 72,000 miles.

Thanks!