Windows 7 won't start up

I have a dell Optiplx GX620, with windows 7 professional.  The PC say's it was designed for windows XP on the front, so it must have be upgraded to 7 at one time. I have tried the free upgrade to windows 10, but it won't install it.

When I start up my pc it goes to
Alert! OS Install Mode Enabled. Amount of available memory limited to 256MB.
  Strike the  F1 key to continue,  F2 to run the setup utility.

I have no windows disc to install, so I have used the F1 "Start windows normally"  It say's starting windows, and then goes to a blue screen, then back to "Strike the  F1 key to continue,  F2 to run the setup utility" (The blue screen goes away so fast i can't read anything on it.)

 It has done this in the past, but would start windows with no problem. The pc started to give me other problems while using the pc. It would go away (blank) the n come back like nothing happened, or freeze (cursor would disappear) then come back and continue on.

My neighbor gave me this pc because of all problems with my XP computer. He thought it was a memory problem, so I replaced the sticks, with new ones. But it did nothing for it. Now I can't get to boot up.

I am on my old XP right now, that you tried to fix for me, but I gave up, when my neighbor knew I was having problems, and he gave me the other one with 7 on it.

 Pat

Thanks,
I went into the OS install mode, and it was on, I worked at it a couple of times, then it finally stayed off, it kept resetting it to on, but it is finally off.

It has rebooted, and windows is working (for now!) I hope it stay's this way. If not, I'll be back!

I guy I got this pc from bought 8 or 10 pc's at once, from I believe his work place. They were all cleaned of their info, and checked that they worked first.

Thanks again! Pat

Perhaps it has something to do with the version of Windows 7 that is installed. There were 6 different versions of Windows 7.