THIS IS AN IMPORTANT WARNING:
With IOTA becoming popular, people want to store their coins in a local wallet. Since IOTA requires a seed, people just randomly click on any web-site link to create a seed.

Don’t be mislead by nice-looking websites which produce QR codes and pass off as the official IOTA website. Fact is, that you should not use any online resource to create a private seed.

It will be just a matter of time until someone will put up a malicious website, posing as a seed-generator, harvesting the seeds. THIS IS A HUGE PROBLEM – READ ON:

What is the IOTA seed?
Think about your seed as the combined username and password that grants access to your bank account. If anyone obtains your seed, they can login and access your funds.

You must generate a random 81 character seed using only A-Z and the number 9. It is imperative that your seed be an 81 character random assortment of A-Z and 9.

I am not sure how you can do this on Windows, but on Linux / OSX it is a simple command:

OSX:
cat /dev/urandom |LC_ALL=C tr -dc 'A-Z9' | fold -w 81 | head -n 1

Linux:
cat /dev/urandom |tr -dc A-Z9|head -c${1:-81}