Sorry to hear you have a bad copy of Descent 1.5. In order to download the copy of DESCENT.SNG at the 3D Realms Forums, you do have to be a registered user and logged in. I just tested the download, and it works fine.
For better or for worse, you can't post downloadable material on this board, but since the DESCENT.SNG file is so small, I thought we might resort to an ancient Unix technique for transmitting binary files, namely uuencoding/uudecoding. I am placing a uuencoded version of the DESCENT.SNG file at the end of this post. In order to generate the DESCENT.SNG file, you will need to do two things:
1) Cut and paste everything between and including the 'begin' and 'end' below into a file. Name the file whatever you want, although you might need to alter the filename extension to something else in step (2).
2) Find a UUDECODE program compatible with the operating system you are using and execute it against the file you created in step (1). You might need to set the filename extension to some particular value in order to get your decoding program to work (consult your documentation). Here is the Wiki for uuencoding/uudecoding:
http://en.wikipedia.org/wiki/Uuencode
At the end of the Wiki you will see links to a couple of open-source programs that can be used to perform the uudecoding. I understand that WinZip can be used for uudecoding, and I am sure Google can provide you with additional programs that can be used for this purpose.
If you use the procedure described in the 3D Realms post to fix your Descent 1.5 installation, please post your results here. Nobody has ever provided feedback on the procedure, and if it actually works, I might be willing to write a patch program to automate the fix. It would also be helpful if you could post which of the disk versions you have (CD-ICD-500-0, CD-H95-700-3, etc.), particularly if you have a version which is not listed.
begin 0666 descent.sng
M9&5S8V5N=\"YH;7`):&%M;65L;RYB;FL):&%M9')U;2YB;FL-\"F)R:65F:6YG
M+FAM<`EM96QO9&EC+F)N:PED<G5M+F)N:PT*96YD;&5V96PN:&UP\"6EN=&UE
M;&\\N8FYK\"6EN=&1R=6TN8FYK#0IE;F1G86UE+FAM<`EI;G1M96QO+F)N:PEI
M;G1D<G5M+F)N:PT*8W)E9&ET<RYH;7`);65L;V1I8RYB;FL)9')U;2YB;FL-
M\"F=A;64P,2YH;7`):&%M;65L;RYB;FL):&%M9')U;2YB;FL-\"F=A;64P,BYH
M;7`):&%M;65L;RYB;FL):&%M9')U;2YB;FL-\"F=A;64P,RYH;7`):6YT;65L
M;RYB;FL):6YT9')U;2YB;FL-\"F=A;64P-\"YH;7`);65L;V1I8RYB;FL)9')U
M;2YB;FL-\"F=A;64P-2YH;7`);65L;V1I8RYB;FL)9')U;2YB;FL-\"F=A;64P
M-BYH;7`);65L;V1I8RYB;FL)9')U;2YB;FL-\"F=A;64P-RYH;7`);65L;V1I
M8RYB;FL)9')U;2YB;FL-\"F=A;64P.\"YH;7`)<FEC:VUE;&\\N8FYK\"7)I8VMD
M<G5M+F)N:PT*9V%M93`Y+FAM<`EM96QO9&EC+F)N:PED<G5M+F)N:PT*9V%M
M93$P+FAM<`EM96QO9&EC+F)N:PED<G5M+F)N:PT*9V%M93$Q+FAM<`EI;G1M
M96QO+F)N:PEI;G1D<G5M+F)N:PT*9V%M93$R+FAM<`EM96QO9&EC+F)N:PED
M<G5M+F)N:PT*9V%M93$S+FAM<`EI;G1M96QO+F)N:PEI;G1D<G5M+F)N:PT*
M9V%M93$T+FAM<`EI;G1M96QO+F)N:PEI;G1D<G5M+F)N:PT*9V%M93$U+FAM
M<`EM96QO9&EC+F)N:PED<G5M+F)N:PT*9V%M93$V+FAM<`EM96QO9&EC+F)N
M:PED<G5M+F)N:PT*9V%M93$W+FAM<`EM96QO9&EC+F)N:PED<G5M+F)N:PT*
M9V%M93$X+FAM<`EI;G1M96QO+F)N:PEI;G1D<G5M+F)N:PT*9V%M93$Y+FAM
M<`EM96QO9&EC+F)N:PED<G5M+F)N:PT*9V%M93(P+FAM<`EM96QO9&EC+F)N
M:PED<G5M+F)N:PT*9V%M93(Q+FAM<`EI;G1M96QO+F)N:PEI;G1D<G5M+F)N
H:PT*9V%M93(R+FAM<`EH86UM96QO+F)N:PEH86UD<G5M+F)N:PT*&G5M
`
end