I tried to do like you said Spike. and I did find the file with the problem "textures0.pk3".
But when I try to copy the file directly from the CD I get this error "Unable to copy textures0. Dataerror (CRC-error, Cyclic Redudancy Check).
>8'(

clean the cd try again,
copy in pure dos if u have 95/98
try disableing dma on cd drive.

or take the disk back and get a replacement.

About the unpure client/invalid pk3 files:
1: It could be that you disconnect from the server, and in a short time again reconnect to the same server. I see people getting these messages quite often when they just disconnected from server.
2: autodownloading on. You said you already have it.

I ain't sure about this, but maybe when you start sof2, you start without a mod. But choosing an OSP server could make you use a different sof2mp config file, because all mods have different sof2mp.cfg's. So in stead of reinstalling the game, (the very last thing you should try) I advice you to search on your computer for all sof2mp.cfg's. Allowdownload is in my sof2mp.cfg's "0" and i never get unpure clients/invalidpk3 errors.
Let me know how it worked out please, because i'm running custommaps on a OSP 3d server, and i see these errors very often.

Do what Spike said.

Well I'm starting to suspect you CD-rom more then anything. I don't know what kind of system you are running. But if you got a second CD-rom, CD-RW, DVD-rom.. etc try that.

If a friend of yours has a PC, you can ask him to let you test the CD there. try to copy that textures0.pk3 from the CD to his harddrive. If that succeeds it is deffinitaly your CD-rom(or windows)

http://www.gamingforums.com/showthread.php?s=&threadid=73649
Some1 with the same problem as you. Check the solution.

I noticed that when you're disconnected for Impure PK3s, if you check back in the console, there should be a record of which pk3 triggered the error.

wow someone with the same problem at last! well i didnt have downloads on so i put it on and it seems ok now although the server ive been trying to get on still wouldnt auto load what i needed and told me to go get it manually.i did have this prob on some of the standard gold maps (jordan1 and kam5) but reinstalling sorted that.after that it was just some of the custom maps.ive only tried a couple of servers after enabling downloads so i may be back! good luck m8.incidentally,my prob was nuthin to do with reconnecting to servers straight away and wiser 2 found no corrupt files.

yep yep yep it workied for me!