The noise is more than likely down to a fan. Remove the side of the case and give it a good dust out paying perticular attention to the CPU heat sink and fan assembley do not use a hoover as this causes static and the components do not like this. Also it could a bearing drying up on a fan.
The slowness could be down to many problems eg overheating, a fault with memory/hard drive/mainboard etc or a software issue.
The easiest to do is the dusting out. If the machine is overheating the CPU will throttle itself to prevent damage while the mainboard will be running the CPU fan at full trying to bring the temperature down.
Get yourself to Maplins, Pc World or an independent dealer for a can of compressed air or if you have a low pressure compressor you can use that (the hobby type, the others contain oil), just do not use a hoover.
If this dosent solve the problem then you will need to look further into it, noise im not to concerned about at the moment. A memory paging issue, bad sectors on a hard drive, main board controller fault etc can all cause a machine to run slow, download and burn to a cd Hirens Boot CD
http://www.hirensbootcd.net/download.html of which has some very good hardware testing utilities and is bootable so you dont need to run windows to use it.
If all else fails and hardware tests ok then wipe windows and start again, this should be last resort really as you will loose all data on the machine unless you have a back up.