You can force boot in safe mode by using msconfig:
Click on Start, then Run.
In the Run dialog box, type
msconfig and press Enter.
The msconfig utility will now launch.
Click on the Boot tab. Among the checkboxes at the bottom under Boot Options, click the checkbox next to Safe boot and select Minimal (if you really need network access while you are in safe mode you can select Network instead). Click OK.
Reboot the guest, and it will boot into safe mode. Once you have completed the installation, you need to reopen msconfig as above and uncheck Safe boot, to boot next into normal mode.