XP hangs after 2-3 minutes of booting/during high activities

My Windows XP SP2 tends to hang a few minutes after booting – i.e. while AVG and ATI CCC are still loading. It seems that hanging occurs mostly when there’s high level of activities/CPU processing going on. For example it tends to work fine while playing movies using DVD player. But if i play some videos via online streaming it will hang frequently. I did clean the fans and heatsinks and dont think its due to heating problem. I did initially suspect HDD related errors but the HDD works fine in other PC – no bad sectors also. Really appreciate any pointers to resolve this.

Thanks.

Regards,
Joaowee