Is there a difference between big chainring and big sprocket, and small chainring and small sprocket? I am using a road bike as commuter its and old bike and have weird front gear selection 52 and 42.

So I took it to work this morning and thought that for a change I will ride it on big gears. While I was getting more tired and lowering the gear I had a thought is there actually a difference between going on low front and back gear and going high front and back gear, but both having the same gear ratio.
Depends - you are cross chaining when in the big/big and small/small combinations.  See this question:

How bad is cross chaining?

Also, theres a question of ratios, but this is more theoretical.

Assuming your rear cassette ranges from a 12 to a 28 tooth, and the front chainring is 52 and 42, then Sheldon says these are your Gain Ratios.

 Gear chart using Gain Ratios
 For 700 X 28 / 28-622 tire with 12-13-14-16-18-21-24-28 8-speed cassette

      52        42    23.8 %    
 12   8.7       7.0   8.3 %
 13   8.0       6.5   7.7 %
 14   7.4       6.0   14.3 %
 16   6.5       5.3   12.5 %
 18   5.8       4.7   16.7 %
 21   5.0       4.0   14.3 %
 24   4.3       3.5   16.7 %
 28   3.7       3.0   


so expressed another way, here are your overlaps, where 1 is the biggest cog and 8 is the smallest.  (42/13 is almost the same as 52/16)

 52       1   2     3   4 5   6 7 8
 42   1 2   3   4 5   6   7 8


So if your comfortable speed for a road is in the middle of the range for 42 tooth chainring, its at the bottom of the range for the 52 tooth.

To say that another way, the 42 tooth chainring has more ratios closer together in one are, at the cost of having no ratios elsewhere in the range.  



So on our hypothetical 8 speed there are two gears almost identical, 42/13 and 52/16.  Why choose one over the other?


Cross Chaining - the smaller chainring will be "happier" with the cassette at 1-5 and the larger chainring will go better with 3-8.  There is overlap.
What's coming up - if you know theres a climb, a descent, or an obstacle/stop then you might choose to be in 42 for the climb, or in 52 for the descent.  For a stop, you'll probably change gear, 42/13 would be a bit hard to pull away from a dead stop.


If everything else was the same, I'd choose the bigger pair over the smaller.  More teeth in contact with the chain means slower wear.

Play about with Sheldon's calculator at http://www.sheldonbrown.com/gears/