quick question

[KSiG]

Does a motherboard come with a operating system installed ....and if not can i install one with a external burner

[_dangtx_]

some have a flash thingy with linux on it or you can just pop in your favorite distro
and use it like that or install it to hdd.

[Silvgearx]

All mother board comes with pre-loaded BIOS if that is what your asking.

[frontier204]

Motherboards don't come with enough internal memory storage to hold a full OS, although they obviously have BIOS or UEFI that can load an OS from internal / external CD-ROM drives, internal / external hard drives, or USB flash drives (although the last couple BIOSes I got were very temperamental in trying to load an OS off a USB drive).
I installed Windows 7 using an external DVD-RAM drive before. You just have to set your BIOS to boot from external CD-ROM (sometimes shown as USB-CDROM or something in the options) and you're good to go.