You can ignore it - many of the letters we received are from MS Solution providers - which actually come in and do a complanicy run on all your machines. So they charge you for the complaincy report - then they charge you for additional needed licensing... They get a double bang for a Letter...
The only legal way they can walk into your offices - is if they have a warrant for a search. They can ask - just like a policeman can ask to search your car. You say "Yes" or "No" - if you say no - he has to get a warrant - and provide evidence of why they want to search your vehicle...
Don't be foolish - ignore the letter - but also don't be foolish - check your complaincy to software... esp if you have internet .. loads of people download evaluation software and just keep using it..
1) get from purchasing all software / hardware purchase PO's
2) there is software out there that will audit your equipment - some of its cheap... or send staff around to audit the machines..
3) compare - and get in complaince ... with MS and any other software..
When you do it by human hands - people trend to skip steps - electronic is better - spend the money on software rather than external services...
Thats my recommendation ... Ignore the letter - but make sure your complaint...