Hi After a painful automated upgrade that failed to successfully install Windows 10, I have finally managed to get it working. The way I did it was to boot the install into safe mode and uninstall the intel display adapter and the touchpad apps. By using the basic windows display driver I then had a stable system to use, albeit with a poor display. I then flashed the bios to the latest version and did a driver update for the display adapter which seems to work fine. I don't know if it will work for others but the laptop restarts and seems stable. The fingerprint reader and Bluetooth aren't working yet but I will try them at a later date. Hope this info helps and at least we know W10 does work :) Paul
I found this list on the Dell site that tells you whether they have tested a particular model for its ability to be upgraded to Windows 10.   The Dell Vostro 3550 is not on the list of about 20 Vostro models, some with lower model numbers ,  some higher.   Will they ever test it??    If you try to upgrade,  windows 10 copies files and then in the 2nd of the three major sections of the upgrade (where you have that big circle in the middle of the screen)  it just goes to a blank screen and then all disk activity ceases and it becomes well hung.   Of course now, the machine thinks its os is win 10 and you cannot go back to a win 7 recover point .  So you just lose all your data and apps and restore the factory image and then install 208 windows 7 updates and it takes like 8 hours in all.  Sounds like a similar problem that people had with Vostros going from win 7 to win 8.   Why not fix it.   
Windows 10 Upgrade - why isnt Dell Vostro 3550 on the list from Dell ?