I installed Windows 7 Ultimate 64-bit, but unfortunately some of my software will no longer work.
Is it possible to change a 64-bit system to 32-bits?
Sorry friend! There is no way to change 64bit To 32bit.
But there is a way to install softwares which are not supporting 64bit OS.
Just run them in a compatible mode and install.
Which of your software will not run on a 64 bit Windows system?
I agree with Hovsep and Jan. You can't "change" the system. It would have to be a complete reinstallation, and you'd need to check your discs & licence to see whether you can do that.
Jan has covered most of the technical points. Typically, the only 32 bit software that won't run on Windows 64 bit is stuff that runs close to the hardware (device drivers and antivirus packages are the most common examples). If your software won't run on your Windows, try to get to the bottom of it. Don't assume it's just a 32/64 bit thing.
As Hovsep said you cannot just switch from 64-bit to 32-bit Windows.
1. This step requires a full re-installation of Windows.
2. This is only possible with a retail license. If you have a OEM license e.g. shipped with your computer the license key is not valid for a 32-bit installation.
As for your software not working I wouldn't even guarantee that switching to 32-bit Windows will solve this. It may just be a problem with Windows 7 itself.
Windows 7 shouldn't have any problems running 32-bit software on a 64-bit installation, it as a very capable 32-bit emulation system (WoW64). That's how Chrome runs on a 64-bit installation, that's how Steam runs, that's how most games are (still) being run. Microsoft themselves even suggest to run the 32-bit version of their own product, Office, on a 64-bit Windows to prevent compatibility issues with other software.
The one point where the emulation will fail is when your application requires to run in kernel mode as WoW64 itself is only executed in user mode.
But getting back to the topic:
Right-click properties on your application (executable), go to the compatibility tab and try running your application in e.g. Vista or XP compatibility mode. You may also try to "run as administrator.
Since you own the Ultimate edition of Windows 7 you are also eligible to run Windows XP mode. It's basically a free Windows XP virtualisation offered by Microsoft.
I also want to point out that 64-bit Windows (XP) was released over 9 years ago and Windows 7 is also close to it's 5th anniversary. So it may be the time to get an alternative for your software.
if it is retail license then it should work, but first format the hard drive/or partition (do backup your files) containing the 64bit. Perhaps you will need activation via phone. or you can try to uninstall the windows product key first then format the hard drive and reinstall the 32 bit
before anything do image your windows to recover back if necessary.