It's hard to see enough detail in the photos, but it looks like either a Boxelder Bug or one of its relatives (see the linked page). They are a nuisance, as you noted, but not harmful. Like some other insects that overwinter as adults (like some ladybugs), they seek warmth and sheltered spots to escape the cold, so often find their way indoors. Sealing any access points is the simplest way to keep most of them out. (Look for torn window screening, worn-down door or window weather-stripping, and any other gaps/cracks in exterior walls, like around vent or pipe covers, that can be sealed.) Once indoors, they can be caught and tossed back outside.

Miri

Could you tell me any bug repellent/killer?
It's inevitable to open the backyard door for house chores.

Thanks

Repellent won't work well if they are simply crawling/flying in through the door. Bug spray generally just kills them on contact with the dried spray residues (which kills beneficials like ladybugs just as readily as the nuisance species), so if they don't happen to land on or walk over the treated surfaces, a spray won't do much good. We don't have any brand or specific product recommendations; any spray labeled for residential/indoor use which includes a wide list of insects (they may or may not specifically list boxelder bug) should suffice if you feel the need to try one. If you have pet cats or aquarium fish, use extra caution if applying a product using a pyrethroid (these are synthetic versions of pyrethum, a certain type of insecticide chemical), because they can be toxic. There are many pyrethroid chemicals (you can look up the active ingredient name on the product's label to check), but they often have names ending in "-thrin" (permethrin, deltamethrin, cypermethrin, etc.).

Miri

You were very very helpful.
Thanks 

You're welcome.