I installed xp64 on a rig that already have xpro. It runs fine but some of my apps do not work well, so I am trying to get rid off it but I am unabel to find how to do it and I do not want to nuke my entire system. Any ideas?

If you installed XP 64 over XP Pro, then you're likely going to have to do clean install with a full format. If you installed it on a different partition, then just format the partition.

Fortunately I did installed xp64 on a separate partition, I understand the procedure, nevertheless, what about the boot menu? Isthere anything I have to do with the boot.ini file?
Thanks before hand

Been years since I set up a dual boot system but I don't believe you need to do anything to the boot.ini after formatting. If on start up the system is looking for or giving you the option to boot to the OS that was removed, you can delete the respective line in the boot.ini. Had to do this a while back when for what ever reason an installation of XP was interrupted and I did a second install, there was a line in the boot.ini referencing the interrupted install and I just deleted it.

Thanks, thanks, It did work as you said. I was extra cautious because I have a lot of important stuff on my computer. Thanks

Glad it worked for you. Suggest you get a second drive for back up and/or at least partition your hard drive keeping your OS/programs on the C drive and your data on the 2nd partition. Not only will it better protect your data but reduces you anxiety with this stuff happens. If you decide to partition, Partition Magic is an excellent program and you can use it after Windows is installed.

Just remove the xp64bit line from boot.ini

[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS << make sure this is correct
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
remove etc >>>multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Windows XP Professional x64 Edition" /noexecute=optin /fastdetect

Then you should be ok to remove windows/program files folder on other partition.........

msconfig is usefull for general config stuff....