98 se locking up

I have a no name system. it’s got a p4 celeron 2.0 ghz cpu,512mb ram,2.1 and 80 gb hard drive,cd-rom and cd-rw.
the system keeps locking up. it seems to be random but happens most often while on internet. i am using cable modem for broadband access.
i also get LOTS of exception and fatal exception errors that usually cause a reboot. i am up to date at windows update. i run mcafee virus scan and firewall. Please Help!