Thanks for replying, Acres, but can you explain how it worked fine for 7 months on the old pc if it is not genuine?

Thanks Acres, I accept there is a probability I have a pirate copy, but still there are a large number of people who have the same problem with Adobe products who claim to have downloaded them directly from the Adobe site.  I think I shall have to take the bull by the horns and contact customer support.  Thank you to all for replying.

This doesn't ring true ssprengel, I've been doing some research after a totally unsatisfactory online chat with Adobe. The message I get is very specific in saying  'invalid serial number for this product' along with an exclamation mark in a YELLOW warning triangle.  There are various explanations for this online (including from Adobe) but none of them specifically refer to pirated software. I am as convinced I have a genuine copy just as much as Adobe say my serial number is not recognised by their key checking software (they will not say outright that it is fake, just the serial number is not recognised).  By going online and obtaining a different serial number I am able to start cs5 and enter this number to get the green tick and carry on in cs5 until the next time.  The following time I start cs5 I get an exclamation mark in a RED TRIANGLE with the words 'invalid serial number' (without the 'for this product').  If you look for other error messages online there are several which may be of interest to you for your scenario, such as 'this serial number has been deactivated'. I should also point out also that when I enter an incorrect number to fool the program I am immediately kicked into touch the next time I start it, it will not carry on working, as you think, until the next time I have to reinstall it. As I said in my first post, cs5 had been running quite happily on my old pc for 7 - 8 months. The only issue I had then was that it would not let me register it with Adobe, but tis is nothing new to Adobe as there are many many posts online with exactly the same problem ( even with cs6 as posted only a few hours ago on this forum - perhaps someone would like to tell him that should his pc crash he will no longer have cs6!). After mulling it over for a few days I intend to take the product to my local photographic retailer who sells Adobe products (and who also holds classrooms for techniques including in the use of Adobe products) for an honest opinion, something that Adobe refused me.  Also check Adobe's policies on support and you will find they will refuse support from products bought on e-bay and will also refuse support on products bought ANYWHERE at prices discounted more than either 20 or 25% (I shall have to confirm the exact ammount, but it is stated in their policy online). This brings into question the issues of restrictive practice and price fixing which are both illegal in the UK, just ask any bank.  It also brings into question just what support a customer from my local photographic shop could expect from Adobe after purchasing a copy of cs5 for Mac for £399, reduced by approximately £250 ( more than 25%). And where does their policy leave someone who may be lucky enough to recieve it as a gift?