Intel 2600k Overheating

50C idle temperature is too hot. It will depend on your ambient temp though.
You could try reseating the the Heat sink using a good thermal paste like Artic Silver. Their Web site will show you the best application technique for a specific processor.
I also did not like the stock cooler and used a Hyper 212+ (about $30 I think). It can be a bit fiddly to fit and you need to check your Motherboard and case for clearances. There are others of course.

Also here is the link to the Intel Processor Diagnostic Tool (64 bit).
http://downloadcenter.intel.com/Deta...&DwnldID=19792

I like the better cooler idea that has been mentioned. I never use a stock cooler. You will have to do you Google research to make your choice. Their are many O/C websites that could help your research. For air cooling this is what I would use. Noctua NH-D14 review If your temps went up and your haven't done any changes but cleaning things and they went down it sounds like cpu thermal compound and proper mounting of cpu cooler. All the stock Intel coolers I have seen have 4 push and pop in pins that hold the cooler down. Sometimes these pins don't snap all the way into the proper position and will cause overheating. I also use Artic Silver and install it as per their website.
Arctic Silver, Inc. - Intel® Application Methods
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Doing a little research I found people clocking your type cpu to 4.4 running 60c under load on air.

Alex,
Are you overclocking? Yes/No

Be sure that the computer is not flush or even close to a wall on any side.

Layback Bear said:

Doing a little research I found people clocking your type cpu to 4.4 running 60c under load on air.

Interesting I overclock for some video encoding and these figures are pretty much spot for what I get with a Hyper 212+. CPU all threads flat out for a couple of hours. Maybe a bit cooler if it's a cold day.

I've a simple approach.
Speccy shows temps. link to speccy in my sig.

I use this processor.
A aftermarket heatsink is a good idea for it.
If you are at 50-55 idle it's very possible it could reach dangerous temps when under full load.

It's obvious the heatsink you are using is simply not conducting the heat properly.
It could be a factory screw up in the heatsink, it could be due to high ambient temperature where you have the computer.
Either way a aftermarket heatsink is the best solution I have based on what information is here.

The bios idle state is completely different from the windows idle state as far as the cpu is concerned.
Bios data is simply 'powered on'
Once windows loads it actually opens up all the features of the chip.
Turbo also comes into play, though it shouldn't be activating while idle.