I've had the same problem since 14.20, so I was thrilled to see some changes in the new v15.20 ("edit font" used also in "List Lines Containing String" dialog).
But was very dissapointed when I found that the problem is even worse in this new version!!
The boxes/squares instead of TABs is now gone, but the TABs are now skipped completely making the result even more unreadable... And the font used is smaller than the edit font, so when using a small edit font (I use 9 pt) the text is messed up in the search result window. And the letter-spacing is wider (looks weird), line-spacing is bigger (not so much of a problem)
I'm running Vista 64-bit, but I think I had the "squares instead of TABs" problem on my old XP 32-bit also...
I use "ProFontWindows", 9 pt as edit font (free font, popular for programmers: http://www.fontsquirrel.com/fonts/ProFontWindows
I reported it to IDM yesterday, and the issue was forwarded to the developers, so hopefully it will be fixed soon now!
...the squares were irritating, but not fatal, but these new issues ARE indeed fatal and makes the function pretty unusable for me as I often use it in files with lots of TABs...
TABs should be displayed as TABs and nothing else. Who would expect/think of anything else?