[TrafficVault]

In addition to "advertising" you may want to think about building relationships with folks in similar businesses- for example the guys who install networks, telephony,etc. They might be able to generate referrals for you.

[inexorable]

Although you are searching for free advertising, PPC advertising for computer repair can be very cost-effective too, since you are able to bid very low amounts in some regions.

In my experience, the average client found through PPC advertising is willing to spend $120 for a computer repair job, vs. $30 through Craigslist.

[jasondrohn]

Definitely - Craigslist. You can also try to get included in Google Products (as long as your repair service is sold in packages it should be cool).