browser freeze

My brother has reported the following problem to me. When he browses the web, his computer will quite frequently freeze up. The system is a 900MHz Athlon on a ECS K7S5A board with 512MB of PC133. The computer had been running for over 6 months without any problems. This issue has just spring up within the last month. Originally I had built it with 256MB of Crucial ram, but he added another 256MB of generic. I thought this could be the problem but he insisted he had the additional ram in there for atleast 2 months before the freeze ups started. So next I figured it was something with his IE v6. I checked for any updates on the MS web site (none). I then dl'd Netscape for him. Guess what, same problem. This problem doesn't seem to be stricly hardware related... since I can run the system hard with games like Serious Sam, benchmarks, etc and it will be just fine. The temp runs about 45 C, so that isn't an issue. It's only when you surf with a browser that it locks up! I really don't know if this is a Win2k specific problem, but I really don't know what it would fall under. The only other thing that comes to mind would be to reinstall the integrated NIC driver. Anyone have any suggestions or seen something this bizarre?