What happens with EFS-encrypted files, if I install Windows 7?

"Not unless you backup your key!

Instructions:

Click Start
Click Run
Enter 'mmc.exe' and click OK
Click File, then Add/Remove Snap-in
Click Add
Select Certificates and click Add
Leave selection on 'My user account' and click Finish
Click Close
Click OK

Select Certificates - Current User in the lefthand pane of the MMC console
Select Personal
Select Certificates. Your personal certificate information should appear in the righthand pane of the MMC console
Right-click on your certificate and select All Tasks
Click Export
On the Welcome screen, click Next
Select 'Yes, export the private key' and click Next
Leave the defaults on the Export File Format screen and click Next
Enter a strong password, then re-enter it in the Confirm Password box, then click Next
Enter a name to save your EFS certificate export file and browse to choose a destination folder to save it in, then click Save
Click Next
Click Finish

Then import it on your windows 7 installation:

Right click the .pfx file that you exported and press Install PFX.
Press Next and confirm you have the correct file.
Press Next and enter the password for the private key.
Press Next and press Place all certificates in the following store and press Browse.
Press Personal, OK, and Finish.
Press Yes and OK."