at -65dBm you should not experience that amount of packet loss, however; trusting your measurements are correct, there are a couple of things you might be able to do in order to improve without "building a repeater"
If the computer is in a static location, and the router is in a static location (this is usually true in most installs) then you can improve the signal greatly with some handy antenna work.
For the computer, you should get a WiFi card with an external antenna connector, these are not too expensive.
Once you have an external antenna connector on that card, you can use a directional antenna to point at the router. There are various commercial models, or you can home brew.
But before we go there, a little tip in regards to signals in houses/buildings:
It is known, and I have positive experience with this, that circular polarized signals have a better penetration-property then linear (Horizontal or Vertical) polarized signals.
Example: I live in a house built in the 18th-century, with 3-foot thick granite walls. I experienced major problems with WiFi distribution, as the signals simply did not travel through the walls. By switching to circular polarized I managed to get reliable WiFi at 2.4GHz throughout the property !
How did I do this:
1) changed the antenna on the router to a "RHCP clover leaf antenna" (you can search for this, there are commercial types, but there are plenty of plans for self-build as well)
2) changed the antenna on the computers which were in a fixed position to a directional antenna, I choose to built a 5-turn-RHCP-helix.
As I now had Right Hand Circular Polarized (RHCP) throughout the property the signals were good enough to use without any "booster/repeater"
I built all antenna's myself, which was a fun project in itself, but I realize that would not be for everyone.
Actually "building a repeater yourself"... I imagine that this would be a very complex and precise project, which you should only do if you are very familiar with the internal workings of such.
If you cannot get the required signal levels with some handy antenna work, I would suggest that you buy a "WiFi signal booster", they are really not that expensive.