Apologies for the late reply; very busy at work this week, and I haven't had a chance to run all these tests until now!
Both Zonealarm and the hotfix were uninstalled.
I tested the RAM sticks as you described. None of them showed any errors on Memtest, either individually or when plugged in together. I let each test go for at least two passes. I realise in an earlier post that I said I had 12GB of this type of RAM; I actually have 8 (I bought 12GB initially, but there was some reason I took a stick out at the time - possibly because of similar issues, I can't remember!). In any case, I tested them as described and it showed no errors.
I did the two other tests you described.
System file check did not reveal any integrity problems.
I'm not sure what information you need from CheckDisk (or if the logfile is published anywhere), but the screen showed the following when nearing the end of its run:
File Verification Completed:
280 large file records processed
0 bad file record processed
2 EA records processed
44 reparse records processed
CHKDSK is verifying indexes (stage 2 of 5)...
225180 index entries processed.
Index verification completed.
0 unindexed files scanned
0 unindexed files recovered
CHKDSK is verifying security descriptors (stage 3 of 5)
177408 file SDS/SIDS processed
Security descriptor verification competed.
23887 data files processed.
CHKDSK is verifying USN Journal...
34708280 USN Bytes processed.
USN Journal verification completed.
CHKDSK is verifying file data (Stage 4 of 5)
177392 files processed
File data verification completed.
CHKDSK is verifying free space (stage 5 of 5)
79 percent complete (11653727 of 15427522 free ...
Speccy snapshot is here: http://speccy.piriform.com/results/3GGMkOrsFTGsuBcU7OYNoy5