I had the same problem (non-Steam) and took the following steps:
1: Uninstall all SSE programmes, except saved missions (using Glary utilities)
2: Updated Microsoft Visual C++ (as described by Torslunde, with thanks!)
3: Updated Windows XP as well as Nvidia drivers
4: Downloaded fresh copies of all SSE programmes (to eliminate any corrupt install files)
5: Disabled my Avast antivirus for 1 hour
6: Installed SSE 1.35 (tested, works OK)
7: Installed SSE Ferry add-on package (tested again, works OK)
8: Installed SSE 1.4 upgrade (tested again, works OK)
9: Installed SSE 1.5 upgrade (tested again, works OK)
So far so good...