Please advise on the feasability of using the incomming earth on my
desktop as a ariel.Cards these day operate at 850mhz.How should I go
about impedance matching the ariel output of the card to earth
input.The reason I want to use earth as an ariel,as follows,my project
involves using my PC as a survielance monitor enclosed in a solid
case.Using the earth would be very beneficial.Please advise.

Could you clarify your question? ?Cards these day operate at 850mhz?.
What cards are you referring to?

Whilst any conductor can theoretically be employed as a radiator,
there is no reasonable way to connect a RF output to an earth lead for
incorporation as an aerial. Impedance matching, even extremely badly,
is simply impossible.

Best

To clarify,thankyou for answer,the card I am talking about is the 
   Belkin 802.11g Wireless (Wi-Fi) Desktop Network Card.My idea would
be to use existing ariel inside the machine and see if I can receive a
signal.Or somehow make a more efficient ariel and use that inside the
machine and see if that works.Please advise.

I assume using the provided ariel from the card in the machine would
not work.Therefore perhaps modifing the machine in such a way as to
accommodate a ariel will work.The machine has platic front,with metal
chasis behind,,maybe putting some ariel between these two will
work.Please advise.

I think you?ll find that 802.11G is 2.4 GHz. Very tricky.

http://www.proav.de/index.html?http&&&www.proav.de/data/wi-fi.html

I don?t know which card you intend using. PCI? Unfortunately, any
modification to the aerial or the feeder will severely worsen the
output. Aerials are deceptively simple but the design is critical to
the equipment which it is coupled to. If it is designed to stick out
the back, then that?s how it?s got to be. It is possible to remove the
aerial and relocate it somewhere else, even inside the case if there
is no metal screen in the way but it?s a pretty specialist area of
knowledge. Without a field strength meter and a good dose of theory,
you couldn?t know how well it was working or even how to modify it to
get best results. At worst, the output would fry.

What further thoughts?

Best

That does it,I beleive yo when you say messing with mhz ariel is next
to impossible,thankyou.

Maybe your red herring is thinking that you must use the earth as an
antenna?  Is your "solid case" a conducting material (e.g., metal), or
a dilectric (e.g., plastic or fiberglass)?  Are you burying this case
in the soil, or does it remain exposed to the air?  Do you simply need
to make your case watertight, for example, in a NEMA enclosure?  In
such cases you could still deploy a wi-fi card (yes that's 2.4 GHz,
not 850MHz), but via the trick of having an externally mounted 2.4 GHz
wi-fi antenna, such as a small patch antenna, that is coaxially
coupled to your card (get a card with an external RF connector); or
simply passively reradiates the signal within the box by coupling it
(via coax through the box wall) to a second wi-fi antenna therein.  I
have not idea what range or power level, or directionality of signal
you need, so these ideas should be swallowed with a grain of salt.