Sometimes when you try to upgrade to Windows 7 from Vista, the upgrade process stops at as close as 64%. The problem may be due to iphlpsvc service that hangs during the process. But don’t worry there is a workaround for this error.
1) Stop the installation and reboot. You will be booted into vista.
2) Press start and right click the computer to start “Advanced system settings”. Alternatively you can also do it by pressing win key + pause break and selecting advanced system settings.
3) Click “Environment Variables”. Now under under “System Variables”, click New and hit Enter.
4) Enter the following details -
Variable Name: MIG_UPGRADE_IGNORE_PLUGINS
Variable value: IphlpsvcMigPlugin.dll
5) Start the upgrade process again.
This time hopefully the installation process will be completed successfully.
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I recommend you to do a fresh install. Its a better option. Upgrades can stop sometimes as you mentioned.