Microsoft Antispyware is just a spyware blocker...it is not designed to act as antivirus software. While it does pick up the occasional virus/trojan, you really should have some other AV protection...Norton, McAfee, etc. However, you could always opt to use a free online scanner from one of those companies. While you'll be missing "on-demand" protection/scanning, you'll still be able to remove any viruses you've picked up.
But, you NEED a firewall, otherwise you are an open book...a hacker could send you a virus, copy your files, read your e-mails, steal your personal info, etc. An AV program is like a second line of defense, and antispyware a third...without a firewall you're ignoring the most glaring hole. You can download a free trial of firewalls from each of the major companies (McAfee, Norton, TrendMicro, ZoneLabs, etc), but finding one that works with your ISP should be your number one priority.
Hope this helps,
John
Hi - I just installed the beta version of Microsoft anti-spyware and disabled Norton which seemingly caused the comp to freeze frequently (see my thread called "freeze" on the XP forum). Is it adequate, and can I continue not to use the Norton AV? By the way, the ZoneAlarm firewall recommended by most is not applicable to my case, because it blocks my ISP and their thechies do not have a solution. Thanks for any suggestion and comment. Amos.

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