Can access https websites but not http sites

Do you use a proxy server ?
If yes check the advanced settings
Tools/internet options
Connections/Lan Settings/Advanced (next to the proxy server)
Check that the http part is correct.

No, we do not have a proxy server.

does this happen to only one user or only one machine

try using a different login from this machine .
also try assigning the same ip to a different machine & see if he can go to http , if not probably it could be some setting on the firewall

This happens on one PC only.  The problem seems to be with Windows Firewall.  When I try to change the settings for Win Firewall, I get this error:  "Due to an identified problem, Windows cannot display Windows Firewall settings."  When I look at Services, Windows Firewall is not listed.  (Note: the Hard Drive is 96% full on this PC.)  I have run Disk Error-Checking during startup on this PC, but that did not help.

That should read:   "Due to an unidentified problem, Windows cannot display Windows Firewall settings."

I did get the Firewall service re-installed and working, using the instructions in MS Knowledgebase article 920074.  Then I disabled the Firewall service.  But, I still can't get to http: web sites (https: sites work fine).

try this
http://www.techbytes.ca/techbyte23.html
it explain a similar problem

also try
http://support.microsoft.com/default.aspx?scid=kb;en-us;813444

Thanks for those comments;  however, those articles tallk about a problem getting onto https: sites, I have the opposite problem.  I can get onto https: sites, but cannot access http: sites.

I ended up re-formatting the Hard Drive, and re-installing Windows XP.  Now it works fine.
Thanks for your suggestions.