Just found out that our proprietary software cannot run on (NT Wkstn) SP6 or 6a, must have SP5. We are at 6a, anyone know how to get back to SP5 ? No uninstall feature on add/remove either?

Sorry, but Service Packs are permanent. The only way to get rid of them is to reinstall NT.

That is absolutly not true. Why do you think it aks you if you want to backup before you install a new service pack. If you chose to backup the data during your install of 6a, Look in your Winnt folder. The first entries will look something like $NtUninstallQ123456$ Run the latest one and it will un-install the latest service pack.

Yes and No. In SP6a particular, the answer is No. From Microsoft's website: SUMMARY When you uninstall Microsoft Windows NT 4.0 Service Pack 6 (SP6) or Service Pack 6a (SP6a), some files are intentionally not removed or reverted back to the version prior to SP6 or SP6a. MORE INFORMATION Most of the files are not removed because changes that are made to Security Accounts Manager (SAM) and the security registry hives that are not removed require the SP6 or SP6a version of the files. You must not remove the files after SP6 or SP6a is applied unless you fully restore your computer to its previous installation of Windows NT 4.0. The following files are not removed after you uninstall SP6 or SP6a: Certmgr.msc Comcat.dll Comctl32.dll Crypt32.dll Cryptdlg.dll Cryptext.dll Cryptnet.dll Cryptui.dll Dssbase.dll Dsssig.exe Enhsig.dll Initpki.dll Lsasrv.dll Mscat32.dll Msoss.dll Mssign32.dll Mssip32.dll Msv1_0.dll Odbccp32.cpl Odbccp32.dll Odbcint.dll Pcomdrv.sys Psbase.dll Pstorec.dll Pstorerc.dll Pstores.exe Regsvr32.exe Rsabase.dll Rsaenh.dll Rsasig.dll Samlib.dll Samsrv.dll Schannel.dll Secauth.hlp Services.exe Shfolder.dll Softpub.dll Spmsg.dll Syskey.exe Winlogon.exe Wintrust.dll Wldap32.dll Xenroll.dll There are issues for almost every other SP that prohibit a "clean" uninstall. We have found that it is better to reinstall NT and then reapply SPs as you go. Of course, if the "asker" feels like winging it, that is their option.

"NOTE: You should create the uninstall folder each time you reapply the Service Pack. The Uninstall folder is used to restore your system to a bootable configuration if the need ever arises. "We highly recommend that you leave the uninstall folder on your hard disk even after the Service Pack has been installed and is verified to be working properly. You may install programs, drivers, and so on, at a later date that may not be compatible with the Service Pack driver, thereby causing you to have to uninstall the Service Pack. The uninstall folder is hidden by default and is located in the following folder: %SystemRoot%\$NtServicePackUninstall$ ..." Quoted from this link: http://support.microsoft.com/support/kb/articles/q165/4/18.asp?LN=EN-US&SD=gn&FR=0&qry=Uninstall%20a%20service%20pack&rnk=11&src=DHCS_MSPSS_gn_SRCH&SPR=NTW40