In this paper, we measured the effectiveness of Microsoft’s popular Windows XP in handling the TCP-SYN based DDoS attacks. It is found that a victim computer with 3 GHz Pentium processor running Windows XP without SP2 upgrade, stalls or crashes easily under low bandwidth TCP-SYN attacks in a fast Ethernet environment as 100% of its processing power was exhausted. Whereas the Windows XP with SP2 was found to be more effective in mitigating the resource exhaustion caused by TCP-SYN attacks. The upgraded SP2 features were more successful in preventing the computer crash under such TCP- SYN attacks. In this paper, we investigated why the SP2 was effective in mitigating the resource exhaustion caused by the TCP-SYN attacks on victim computers with Windows XP. This was found to be mainly due to SP2’s ability to limit the number of half-open TCP connections that were active on the victim computer at any given time. Limiting the number of half-open TCP connections also limited the rate of resource exhaustion for the victim computer. We also found interestingly that the firewall (which was designed to defend against other security attacks) when enabled for the Windows XP with SP2, enhanced the bad effect of the TCP-SYN attacks and hastened the exhaustion of the computing resources under such TCP-SYN attacks.