Thanks for your fast response!
This morning my computer crashes/froze again, so I followed you steps:
8/26/13 10:55:47.000 AM kernel: NVDA(Private): Channel exception! exception type = 0x20 = PBDMA Error
8/26/13 10:56:07.000 AM kernel: NVDA(Private): Channel timeout!
8/26/13 10:56:07.000 AM kernel: NVDA(OpenGL): Channel timeout!
8/26/13 10:56:28.000 AM bootlog: BOOT_TIME 1377507388 0
These lines correspond with the time exact time my computer crashed, so I think this has something to do with it.
The last crash log I could find was from a few days ago and I don't know if it is of any importance. Also the log is very long, so I decided to post only the first lines:
Code Type: X86-64 (Native)
Parent Process: launchd 
User ID: 0
Date/Time: 2013-08-20 22:45:51.423 +0200
OS Version: Mac OS X 10.8.4 (12E55)
Report Version: 10
Crashed Thread: 3 Dispatch queue: com.apple.metadata.spotlightindex
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000bad
VM Regions Near 0xbad:
__TEXT 000000010ce5a000-000000010cf7a000 [ 1152K] r-x/rwx SM=COW /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/Metadat a.framework/Versions/A/Support/mds
Application Specific Information:
[0x110660000]/SourceCache/Spotlight_frameworks/Spotlight-707.11/index/common/sto rage.c:689: failed assertion 'success' /.Spotlight-V100/Store-V2/2383C498-EA36-4BD7-808C-D02B90486547 (hfs, t: 0x11, st: 0x1, f: 0x480d000)
[0x110660000](Error) IndexCI in _Bool _storeageSetFreeListHead(storage_t *, int, offset_t):offset(196883) < freeRegion (147712)
[0x10ec34000](Warning) IndexQuery in virtual bool SISearchCtx_FSI::performSearch(struct PartialQueryResults *):Retried searchfs 4 times
Also I forgot to tell this: the crashes/freezes also happen on WIndows 8, which is installed on the same MacBook Pro, so I think the problem is not software-related.
I hope you can do something with these logs