One of the lads that works for me has been having issues with his computer for a while, I felt quite bad for him as he'd spent around £1000 on it and had his friend put it together.
I brought it home, booted it up, did a the usual virus/malware scans, started HWMonitor up while running 3Dmark and saw some pretty extreme temps. He's running a R9 290 so it was a bit of a given for that to be hot but the CPU was idling at low 50's.
I turned it off, took it apart and this is where the fun began...
http://imgur.com/a/5s716
I didn't get it as perfect as I would of liked but temps went down dramatically.
To anyone thinking of building a PC, a pea size blob of thermal paste on the centre of the CPU is all that's needed anything more (as per the album) and your asking for trouble. Also ensure you have some good airflow, cold air coming from the front and/or bottom of the case with hot air going out of the top/back of the case, this should also help to keep dust levels down in the case as it will be sucked in and blown out through the airflow (not all of it unfortunately).
Also, consider paying someone to build the computer for you if you aren't confident in having a go yourself. Make sure to do plenty of reading before hand if you do decide to have a go.
If nothing else I quite enjoyed rebuilding it. Oh and another tip from this build, don't use sellotape to attach your HDD to the case... Yes this did really happen