First thing to do with a long unused computer is to buy a new coin cell battery and install it. 

Dell OptiPlex 760 Service Manual

That computer is very old so if that doesn't work recycle it.

After replacing the battery, you might want to update to Win10.  Your 760 will run it.  At least 8GB RAM is ideal over 4GB.  With an SSD, it'll fly.  My previous 755 did.  You're really in luck if it has a quad CPU.

If you want to keep Win7 on it and it's not working after replacing the battery, the caveat with Win7 is, as we know, updates are no longer.  So I'm not suggesting a Win7 reload.  One of our Rockstars (or community members) might know a way to download Win7 ISO with the updates it came with, but I don't remember what it was for sure and won't suggest unknown site.  Win7 with just SP1 is too raw to do much of anything with it.  Internet hardly works on it and basic features like background colors don't work.

Even with a dead battery in 760 which I have had plenty of similar era models (745+755+760 etc.) you can still clean install either Win 7 or 10. If you still have win 7 dvd you can try wipe the hdd and attempt a clean win 7 os install, albeit the os will be quite outdated (although still looking fresh).  If you like to run on it for a bit then decide if you want to go to Win 10 or not.  Some 7 users really dislike 10.  You may or may not be one.  At this point since your hardware is good except for a possible old cmos battery you can relaunch win7 by a clean install if you do not care about losing all old data on hdd.

Thank you. I don’t have the windows 7 CD anymore but might need to buy one to get it to run in normal mode. Or I could buy a windows 10 CD if there’s no other way to get it on PC from safe mode 

Thank you very much for the advice. I’ve replaced the batter and I’m happy to run Windows 10 on it but would I need to buy a CD to do this?

I tried downloading it onto USB but couldn’t get it to run. 

Sorry, might be an obvious question but really unsure as I can’t do much on it in safe mode 

Re: might need to buy one to get it to run in normal mode. Or I could buy a windows 10 CD

Win 7 pro disk iso can be legally downloaded for free.  Win 7 needs to be activated 3 days after installation either by genuine COA key or telephone call to automated MS activation.

https://techpp.com/2018/04/16/windows-7-iso-official-direct-download-links/

Win 10 dvd can be created free using Microsoft media creation tool.  If you have a Win 7 pro key you may use that to activate Win 10.

https://www.microsoft.com/en-us/software-download/windows10ISO

@PCGUY7 

Windows 7 8 10 ETC is a dvd.

Region Code may need to be set to 1 AKA usa for WIN10 DVD to F12  BOOT.

10 recovery disc N82E16832350238

Is there any way of downloading and installing from safe mode? Or can I download it onto a USB and run it from where somehow? 

Besides Windows 7 being dead, it's a haven for malware. Best to go with Windows 10.

I think you'll really want to have your Windows 7 product key in hand before you start. If it's not printed on a label stuck on the case, you may be able to find it from Windows 7 safe mode. From the Start menu, search for "cmd" and run it, then run the following command in the command prompt window:

wmic path softwarelicensingservice get OA3xOriginalProductKey

The key should consist of five sets of five letters/numbers. The following is random but it should look like this: R3L65-PKGJ4-HWEQV-8FM9D-SNCX2

The Windows 10 media creation tool can be downloaded here. With it you can create Windows 10 installation media on a USB key, minimum size 8 GB. I don't know if the tool will run in safe mode, but you can run it on another PC (perhaps a friend's?) to generate the media. (Alternatively, you can burn it on a DVD.)

Once you have the installation media in hand, you can install it on your PC. With the power off, Insert the USB key. Turn the power on and press F12 repeatedly until the boot menu appears, where you can select the USB storage device. As @redxps630 says, you can use the Windows 7 key to activate Windows 10.

Win10 DVD that @speedstep linked is what I use.  Caveat is going thru the updates.

I think I read from @redxps630 that the ISO comes with the updates.  Yes, it can be downloaded to bootable USB flash drive.  Be sure to follow instructions on making it bootable.  https://docs.microsoft.com/en-us/windows-server-essentials/install/create-a-bootable-usb-flash-drive 

I wanted to get your above post answered in a certain way, so there might be some overlap with other posts.