OK
couple of new and relevant facts here....
... your man is quite correct ... wireless "should" work ... but in my experience ... your problems are not at all unusual.
Firstly go to your local supermarket /newsagent and buy a copy of the latest "computer shopper" mag (jul2007)
(or lurk somewhere quiet 'cause it's quite a long article).
there is a very good article on wireless - and also how you can use your laptop to "sniff" out good positions in it
To save a little time download netstumbler.
http://www.netstumbler.com/downloads/
follow the advice there.
I can't remember if you've already changed chanels ... but use netstumbler to display all the used chanels and if poss change to 6
(look at the book of words for instructions how).
If you really can't face the disruption of all that wiring
(if you can - make sure it's cat 5e)
look at homenetwork - for just one PC it's not too expensive (compared to wire)
(you could possibly also return the new adapter)
Working remotely isn't easy .... but you mention "long way" between machines .... if you are talking old house "long way" + thick walls may = problem. Can you locate the router upstairs? - or even in attic?