"Install Bluetooth" means different things on different machines. Laptops almost always have the radio built in - it only needs enabling or a driver. Desktops very often ship without Bluetooth at all - no software can conjure the hardware, but a $10 USB adapter adds it in minutes. Step one is finding out which situation is yours; everything else follows from that.
Quick answer
Right-click Start, open Device Manager. A Bluetooth section = the hardware is there; enable it in Settings, Bluetooth & devices (and check quick actions for the Bluetooth tile). No Bluetooth section anywhere, including under "Show hidden devices"? A desktop most likely has no radio - plug in a USB Bluetooth adapter, and Windows sets it up on its own.
Most desktop motherboards without Wi-Fi have no Bluetooth either - the feature rides on the wireless card. No driver can install what is not there.
Airplane mode, a function-key toggle, or a BIOS/UEFI setting can switch the radio off - Windows then shows no trace of it.
After a clean Windows install the radio may sit under Other devices as an unknown entry until its driver is installed.
Bluetooth used to work and vanished - that is a repair case, not an install case; the driver stack needs rebuilding.
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Download Driver Updater Downloads Outbyte Driver Updater for Windows 11, 10, 8 and 7. Free 2-day trial, no credit card required.Software alone - no. Bluetooth needs a radio. The practical fix for a desktop is a USB Bluetooth adapter: plug in, driver installs, done. Laptops virtually always have the radio already - those need enabling or a driver, not new hardware.
Any current 5.x dongle from a known brand does the job for headphones, mice and gamepads. On Windows 11 and 10 the common chipsets set themselves up automatically - no disc needed.
The radio is fine, its driver did not make it into the clean install. Windows Update's optional updates usually bring it; otherwise install the package for your Wi-Fi card's maker (method 3) and the section returns.