Dial up Networking Problems

When first got the computer everything ran fine and
perfect I use the win update and have all the current
updates to XP home including the service packs but however
my comeputer was infected with the blaster worm and it
killed my net every 5 min of connecting well installed the
patch and the cleaned the worm and ran the fix blaster
from mcafee. Well ever since then my internet be bad very
bad try connect get errors ppp doesn't match remote
computer and when it does connect and shows it connecting
and connected I get dns errors and programs like MSN
messenger tells me I am behind a firewall and on rare
occasions I do get through and everything works it only
lasts for an hour or two then goes back to the dns errors
and programs telling me I'm behind a firewall. For info I
reinstalled my modem ran the netsh command my ISP had me
do even created a new xp profile which temp fixed it but
started again after week or too I just have no clue what
to do anymore and no I do not use the XP firewall and my
computer says its not running. Anyone has any suggestions
let me know very appreciated I'm about to totally give up
on microsoft products as my isp said has to be something
in xp as everything is fine on there side.