I had to install MySQL 5.5, few days ago. I have Windows 7 64 bit Ultimate edition, The best system, Microsoft have for desktop, I've thougt.
I was very suprised, I couldn't run some system programs after this installation. For instance systempropertiesadvanced.exe. It's program for setting up virtual memory and profiles and so on. Windows wrote me something like Cannot find program %windir%\system32\systempropertiesadvanced.exe. So I went to cmd console and wrote set windir. Nothing appeared. Like system didn't know this variable. To tell you the truth, I began search, after I was unable to run sound volume program from Windows tray. Anyway Windows looked functional, I didn't search net and forgot this problem.
Today I realize , that I had this problem 2 or 3 days ago ( atleast 2 restarts). So I've thought it was only Windows bad day and everything is OK now. I found out, that windows still don't know %windir% variable. So I began google for solution. I found some sfc utility hotfixes from MS KB. But I found interesting article, that wrote something like , when PATH variable is longer than XXX characters Windows lose %windir% variable. I've thought, what a rubbbish, Windows Ultimate 64bit, MS enterprise operation system ?
Anyway I tried it, I went into c:\windows\system32\ directory and ran systempropertiesadvanced.exe, I found the PATH variable, and at the end of value was some MySQL .... path, I delete it and guess what ? %windir% is functional.
I can't understand, I've thought I'm dreaming. Don't believe ? So try it ...