FredrikL,
Can you please clarify how are you moving from windows XP to Windows 7 ?
There is no need for you to store license file. After installing MyEclipse in Windows 7, activating the license will automatically store the license in .myeclipse.properties file.In Windows 7 the .myeclipse.properties file will be in C:\Users\<user name> folder.
Let us know if you see any issues.