Which alcohol is bad to use when cleaning computers?
You would want to use as clean alcohol as you can get hold of. It doesn't really matter if it is ethanol,  methanol  or isopropanol.
As mentioned in the comments low content alcohols may contain additives that will be left behind as impurities. Antisepctics and such are not suitable at all.
A nice test is to wipe some of the alcohol on a glass plate. Once the alcohol vaporizes you should see none or almost none impurities left behind. If there is oil present you will get that oily &quot;rainbow&quot; effect.
Several users have posted safety concerns on methanol, so don't use that. But: all of these are strong chemicals and should be treated as such. Use gloves, work in a ventilated area, and perhaps stating the obvious: They are all extremely flammable.