Great thanks for Esteban Caillou's efforts to make NPOI runable on .NET 1.1. NullableTypes library (http://nullabletypes.sourceforge.net/) is used in this release.
If you don't want to change the source code, please download the compiled assemblies directly.
If you cannot build it via .NET 1.1, please make sure:
a. you have referenced the NullableType assembily and SharpZip assembly with the correct path.
b. SharpZip assembly is not included in the source code. If you don't have the SharpZip assembily in GAC, please download it via http://npoi.codeplex.com/Release/ProjectReleases.aspx?ReleaseId=33203#DownloadId=89205
After that, if you still cannot build it, please contact me.