Sometimes it works (simple programs, DOS-programs), with most programs it doesn't work.
For example, I can run WSFTP 95 LE from a USB-stick on any PC, I can run Swiss Master 5.5 from a USB stick on any PC provided it lets me run a .reg file to register it (or do it via its GUI). That are two simple programs. And then it doesn't matter if they are on a USB stick or on another hard disk that happens to have some Windows 7 system files on it also.
But I'm sure complex programs like an antivirus won't work that way. They have to integrate themselves in the OS during the installation.

Just install the programs in your XP-partition and they will be fine. And, as shown by the 2 examples above, you may be lucky.

Kees

Kees