please disregard the "fantastic advice" above
windows 7 needs MINIMUM 1gb of RAM to run, you can do basics such as disk clean, disk defrag which are utilities within windows. I would recommend increasing RAM whcih will increase performance massivly, you will have to look what type of ram you have i.e DDR DDR3 etc and pop it in,
the fan : The fan is there to cool the hardware down and to prevent it falling off the board as its that hot. I would say not to use the PC as much untill you have found out what the fan is for, why its stopped and fix it ASAP. if the hardware isnt cool it could get damaged which will cost you more in the future, open the case up, it might be a loose connection or the cable may have come off ( reconnect it)
IF...