Ahhh, forget Mainfest files, this tool will do it for you, tested here on ICQ and works like a charm :D
A friend of Warp2search has made a nifty little tool that makes every program ran in Windows XP have a XP skin. Try it out & feel free to comment.
"XP Style is a small program that will force windows XP to skin any windows program, designed for Windows versions prior to XP. It works by patching the executable, but is 100% safe to do. The program makes a backup of the original file, so if anything goes wrong, you can recover. This program works on 90% of the programs, including Visual Studio, Photoshop, Visual Basic itself, ... The best results are achieved if the program uses the standard windows controls and is programmed in C. Applications that feature custom skinning (Winamp) will not be affected and patched programs will run under other Windows versions, if this was the case before the patch. For this program to work, the patched EXE must contain at least 1 resource. In most (if not all) situations, this is the case.
However, applications created by Visual Basic, programs that check themselves for changes to the exe (like virusscanners) and applications that are compressed by an executable-compressor (like UPX) can cause problems. If the patched program does not run, just restore the back-up." (c) 2001 by [HFL]SpHeaRe.
Download: XP Style Tool
News source: Warp2search