need advice please! Acer desktop

ckaggle

Just before christmas I bought a new Acer desktop from Curry's online, just before Easter I had trouble with the power button it took ages to turn and some times when it did it made a terrible noise and I had to keep switching it on and off until the noise stopped, so it was picked and taken away to be fixed. When it came back having been "fixed" the fan is on all the time and it is a constant hum which i can hear all the time, gradually getting louder and quietening down but still can be heard all the time. I called them again regarding this and was told that it was just the fan and it was normal! I persisted and they sent some out today to look at it, he cleaned the fan and told me it was normal.
When we first got the desktop it didn't make the noise all the time, which is why I think there is something still wrong with it. Can any tell me if they think i am right and maybe a course of action for me to take.
Thanking you inadvance any input would be greatly appreicated.