I own and run a small computer services firm.  

Our primary source of revenue comes from installation and maintenance
of networked computer systems for small to medium sized businesses. 
Normally these systems are based on Windows 2003, but we also provide
Linux systems where they are a good fit and represent good cost
savings.

The company has been running for around 18 months and although we have
earned a very good name for ourselves word of mouth is not spreading
as quickly as we would like it to.  We operate in a wealthy part of
the United Kingdom so there is no shortage of prospective clients.

Recently we have gained Microsoft certified partner status and ?Small
Business Specialist? status and hope that this may improve our
visibility and credibility.

I would like ideas and suggestions for cost effectively marketing in
and around the local area (say a 30 mile radius) as we have been quite
unsuccessful in our own campaigns.  

Innovative ideas are welcome; we would like to stand out from the
myriad of IT suppliers if possible.

Our own schemes consisted of a banner ad on yell.co.uk and a Google
Adwords scheme, neither of which I felt represented value for money or
reached the correct target audience.

Until this point all of our work has been gained by word of mouth and
I would like to catch some of those customers actively seeking a
supplier for their systems.

being that you're in the UK, I don't know if these American ideas
would work, but here are a few:
? create a customer tell a customer promotion-tell your current 
customers that you will buy them lunch/dinner/(whatever you
think they find valuable) for every new customer they refer and
you sign up as a customer.
? create a money-back guarantee - if you can't fix a computer 
problem in one visit, you won't charge them a pence.
? direct mail campaign - target the top 100 businesses you want
as clients and send them a CD with your info and references. 
include a coupon for a free analysis of their system - maybe
something about coming in and doing a virus scan for free.
? sweepstakes are a great way of acquiring new leads but 
most of the time these people do not need immediate services.
? offer your services for free/discounted? to non-profit orgs or 
schools/churches in your area. this is an indirect way to get 
the business world to learn about you.
? partner with the stores/companies where your clients
buy their equipment. offer commissions to their sales
force for referring clients to you. create a promotion
where the most referrals wins something valuable.

don't know if any of these will help, but they are some
tactics I've seen executed that have given good results.
good luck.