I bought a new computer. Some of my purchases are not available for 90 days? Why?

If you use iTunes Match, automatic downloads or redownload an account's past purchases then you can tie a computer or device to an account for 90 days : http://support.apple.com/kb/HT4627 - so you won't be able to do any of those three things on another account until the 90 days have completed.

You can copy your content from your old computer to your new one : http://support.apple.com/kb/HT4527

Or from your backup of library/downloads onto it (File > Add To Library).

If you haven't got a backup then you can try contacting iTunes Support and see if they can remove the association - but doing either of those three things on another account is likely to tie your computer to that account for 90 days instead : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page