Here is a bunch of good information that should help:
Can you describe the existing windows in more detail? I'm going off a basic assumption that these are Aluminum single pane?
Most windows will benefit from some "leak sealing" in many cases these are leaks around the exterior of the window. The window itself may leak as well and here replacing some weather striping may help.
Installing a complete new window would be the best as it will have better seals and may be a double pane. A new install, when done correctly will also be seal between the framing and the jamb.
Add to this post.yes they are old aluminum single pane windows . we don't want to spend alot of monies on them as we will be replacing the hole house when paid off. we have put plastic inside and out to no avail
External "storm" windows may help ...but these can often cost upwards of new windows. When you installed the "plastic" was it the "shrink film" type? that type is the most effective as it generally seals out more air leaks due to the continuous adhesive strip. simply stapling up poly sheeting does not help much at all. When I first moved into my home 20 years ago I built some "interior" storm windows from thick clear vinyl, and some 2 x2 lumber these "frame" where then edged with foam weather stripping. The whole assemblies were then placed into the recesses in front of the windows.
Add to this post...yes the plastic was the shrink type.had problems with it sticking. used all kinds of tape