Junk mail or spam is not usually harmful to your computer though dealing with several hundred messages each day can be an infernal nuisance.
Most internet service providers offer some sort of spam filtering on their mail servers and there are various Spamkiller products including those provided as part of the McAfee and Norton internet Security packages.
The biggest problem with spamkiller services is that they tend to filter out messages that you actually want—for example, only accepting messages from known email addresses ignores the tendency for email addresses to change quite frequently.
Also, the dedicated spammers are generally ‘ahead of the game’ in bypassing the filtering rules so quite a lot of rubbish still gets through.
Personally, I favour the approach of ‘filtering in’ the messages from known contacts by setting-up rules which place messages from specific email addresses into their own folder—this is quite easy to do in both Microsoft Outlook and Outlook Express (revamped as Windows Mail in Vista).
For example, you could have a separate folder for every family member, friend, business contact, etc, which also has the benefit of organising and prioritising your mail.
You can also filter on a number of conditions such as the message size, recipient address and specific words in the subject line or message body.
Outlook has quite a good built-in filter which moves suspect messages into a junk folder where you can quickly browse through the subject lines and delete those which are obvious rubbish (NB Outlook Express doesn't have junk mail filtering though the new Windows Mail does).
This doesn’t capture all junk mail but anything unfiltered goes into the inbox where the genuine mail can be identified and new filtering rules set up, as required.
Don’t bother to add a new rule for every unwanted email as the serious spammers will change the format of their messages on a regular basis to avoid this kind of filtering.
Also, with internet security generally, do try to avoid installing multiple software products to do the same thing—they just slow-down the PC and often conflict with each other.