Any noise at all from the CPU is going to be a fan (the only other possible noise being a slight fizz as it melts and after which you wont get any noise at all ;-)
If it is "pulsating" then it may be a fluctuating power supply. Try a new fan and see if it continues, it it does then PSU may be faulty and if a premanufactured PC the PSU's are not always of the highest quality.....
I have not heard of any virus that would cause you to need a new modem - I dont suppose for one moment he gave you the old one back did he?, not, thought not. In fact, I have never heard of a modem failing at all!!
have seen modems go but usually due to lightning and lack of surge protection but a virus knocking a modem out i must admit is a first worst possible scenario would be corrupted drivers i would suggest finding a new retailer
As you are probably all aware by now that I am new to computers, and also green. I just took his word for the modem.In future I will ask the helproom for advice. I could not get online due to the m/blast worm, thats why I had to take the unit in to the shop.