What's up with my cactus?

Hi everybody,
I'm really not that green-fingered but I have recently been given a cactus and am anxious not to kill it (not least because I'm told it's quite difficult to). So I brought my cactus home, stuck it on my windowsill, and a few days later I noticed it has some brown discolouration on the side facing the light. I swivelled it around, and when I went to water it yesterday the side that was facing the window was a rather nice green. I don't think it was the light that harmed it, but the weather's been a bit more overcast lately so perhaps that protected it. Anyway, what have I done to this poor cactus?
There's a picture of it here:
http://19b.mine.nu:8012/cactus.jpg
Pointers on what kind of cactus I've got, how often I should water it (I've been giving it some once a week), and other such cactus care tips would be gratefully recieved too.
Thanks,
--
Alex Pounds (Creature) .~. Website working group chair
CS2 Student /V\ Socfed committee member
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wrote:

You are probably over-watering the plant which is the fastest way to kill a cactus, especially without sunlight. Do not fertilize at this time.
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On Tue, 11 May 2004 15:32:34 GMT, Phisherman wrote:

After my watering yesterday it's certainly looking a lot less healthy today, so I shall not water it for a while and see if it improves. How often should I be watering it?
Thanks,
--
Alex Pounds (Creature) .~. Website working group chair
CS2 Student /V\ Socfed committee member
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don't know what's wrong with the cactus, but it looks like the pot is leaking a human head.
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