interpreting crash log

Hello,

Today unexpectedly my iMac dispalyed a grey screen asking for restart. A few seconds before this screen appeared I was experiencing problems using the system so I had checked Activity Monitor. It showed some deamon by name screenreaderd as Non Responding.

The crash log is

Code:

Wed May 20 11:16:52 2009
panic(cpu 0 caller 0x00192FD4): "pmap_flush_tlbs() timeout: " "cpu 1 failing to respond to interrupts, pmap=0x536500 cpus_to_signal=2"@/SourceCache/xnu/xnu-1228.5.20/osfmk/i386/pmap.c:4570
Backtrace, Format - Frame : Return Address (4 potential args on stack) 
0x44b638d8 : 0x12b0fa (0x4592a4 0x44b6390c 0x133243 0x0) 
0x44b63928 : 0x192fd4 (0x45f414 0x1 0x536500 0x2) 
0x44b63998 : 0x194126 (0x536500 0x27d0a000 0x0 0x0) 
0x44b63a38 : 0x17406e (0xd1b5 0x0 0x44b63ad8 0x65847f) 
0x44b63ad8 : 0x169e3a (0x5db2578 0x0 0x0 0x1000) 
0x44b63bf8 : 0x16c180 (0x27d0b000 0x0 0x0 0x0) 
0x44b63cd8 : 0x12c0e1 (0x17c6e10 0x27d0a000 0x0 0x1000) 
0x44b63d68 : 0x14b8ac (0x53b8e0 0x2 0x4a10fbc 0x4a10fa8) 
0x44b63db8 : 0x12d17e (0x59e93a0 0x4a10f8c 0x0 0x0) 
0x44b63df8 : 0x126257 (0x59e9300 0x0 0x24 0x44b63ee4) 
0x44b63f08 : 0x1973dd (0x44b63f44 0x0 0x0 0x0) 
0x44b63fc8 : 0x19f3b3 (0x4e849e0 0x0 0x1a20b5 0x3f6e8b8) 
No mapping exists for frame pointer
Backtrace terminated-invalid frame pointer 0xbfffe9a8

BSD process name corresponding to current thread: Activity Monitor

Mac OS version:
9E27

Kernel version:
Darwin Kernel Version 9.4.0: Mon Jun  9 19:30:53 PDT 2008; root:xnu-1228.5.20~1/RELEASE_I386
System model name: iMac8,1 (Mac-F226BEC8)

Any help in interpreting this log and identifying the cause will be appreciated.