+editThrashzone makes a very good point
Backup your system.
If you proceed with installing Win10 PREVIEW without backing up your system - you do so at your own risk
At the very least copy your personal data to a safe location
Did you shrink the volume to free up space?
Please post a screen shot of your Disk Mgmnt.
Disk Management - Post a Screen Capture Image - Windows 7 Help Forums
Windows 10 is very light - about 10 GB on my machine
You could shrink C: by 15 (15362) or 20 GB (20482) and still have room for progams and data
You mentioned games, (they're big) - you might want to shrink C: by 40 GB (40962)
Remember, this is a preview version of Windows - treat it as a test environment to see if your there are windows 10 specific issues with the games you play. If they paly in Win8, there probably won't be any issue in Windows 10.
The error on the boot to USB sounds as though the USB was not created properly.
Try creating the media again - pay attention to the Secure Boot & UEFI information
*** It might be better to create a disc - UEFI media on a USB has a caveat that everyone misses
- the USB must be formatted FAT32!
Once you have free disk space and good install media, things will go well.
Read the entire tutorial and ask any questions first.
Windows 10 - Dual Boot with Windows 7 or Windows 8
To answer your question about time limits:
Technical Preview expires on April 15, 2015
If youíre running Technical Preview, we'll send you notifications beginning on April 1, 2015 to remind you that itís time to upgrade your PC to a newer version.