are You using Mash's HRP or the new built into sfall one?
If You are using the sfall one the resolution is set within ddraw.ini, if you want to switch back to Mash's you need to set:
[Main]
;Set to 1 to enable the built-in High Resolution Patch mode that is similar to the hi-res patch by Mash
;The required settings will be read from the f2_res.ini configuration file of the original hi-res patch
HiResMode=0
and use F2_res_config.exe to reenable the patch, or You're using a russian build which has two game executables fallout2.exe and fallout2HR.exe
the standard one isn't patched only the HR one is. ( however the config tool only patches file called fallout2.exe, so you'd neeed to rename the two files [fallout2.exe=>fallout2old.exe, and fallout2HR.exe=> fallout2.exe] , patch fallout2.exe and then rename them back to what they were). be sure as to what your desktop shortcut points to.