BOAC,
Your question still lacks adequate information. Are you talking about a dual-boot environment?
It depends on whether the applications were installed through / under Win 7 or not. If they were, then the presence or absence of XP is irrelevant (unless you specifically chose to install them into a Windows XP system folder!!).
By install, I mean to create registry entries and / or locate application files in the Windows directory structure.
If they were installed under XP, but accessible to Win 7 using the XP executables / binaries, then removing XP will prevent them from working with Win 7. However, I wouldn't expect many such applications to function properly.
Any application whose installation process is simply to place an executable file (and ancillary files) elsewhere on the disk (without modifying the registry or the Windows directory) will be accessible to any Windows OS with access to that disk (16 / 32 / 64-bit compatibility allowing).
SD