2 Things: Mods and Crash Reports

Ok I've been having issues. Ever since I finally got WinZip I've wanted to download mods, but my minecraft file (It's minecraft.jar but it's just titled minecraft, the file I've been told over 100 times to put the mods in) is corrupted. My friend told me it was from an update a while back, and that it screws it up. If you could help me, that'd be great.

ALSO I've been crashing alot but the latest crash report is from like 2 weeks ago. I hate getting technical, and all this is to me is gibberish, so if you could help me with this here's the crash report, I can still get on but I crash frequently, and it crashed today twice but no crash report.


---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 2/2/13 11:20 AM
Description: Exception in server tick loop

java.lang.NullPointerException
at cf.b(SourceFile:103)
at cf.a(SourceFile:35)
at iv.b(SourceFile:436)
at gm.a(SourceFile:72)
at bec.b(SourceFile:73)
at net.minecraft.server.MinecraftServer.r(SourceFile:762)
at net.minecraft.server.MinecraftServer.q(SourceFile:679)
at bdz.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:613)
at fy.run(SourceFile:849)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
Minecraft Version: 1.4.7
Operating System: Windows Vista (amd64) version 6.0
Java Version: 1.6.0_13, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 306798144 bytes (292 MB) / 518062080 bytes (494 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 3965 (222040 bytes; 0 MB) allocated, 3965 (222040 bytes; 0 MB) used
Suspicious classes: ~~ERROR~~ NoClassDefFoundError: com/pclewis/mcpatcher/mod/TileOverride$1
IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
Profiler Position: N/A (disabled)
Vec3 Pool Size: 662 (37072 bytes; 0 MB) allocated, 662 (37072 bytes; 0 MB) used
Player Count: 0 / 8; []
Type: Integrated Server (map_client.txt)
Is Modded: Very likely; Jar signature invalidated

Thanks alot in advance.