After updating agent to 1.8.7.rc1 (revision 21392) I get the following errors in the log.

5308:20110920:130647.210 PerfCounter 'Jýþ' FAILED: invalid format
5712:20110920:130716.351 PdhLookupPerfNameByIndex failed: [0x800007D0] unable to find message text [0x0000013D]

These may occur several times and at random times the agent may hang for a few minutes (so long as to trigger a "system down" PROBLEM in zabbix) and after a few minutes it resumes work like nothing happened (and triggers a "system down" OK in zabbix)

I have a set of general items for monitoring CPUs like "perf_counter[\Processor(X)\% Processor Time, 300]" with 0<=X<=7. Of course not all systems have 8 CPUs, they may have just 4, as is the case with the server in question, and perf_counter instances for CPUs with X>4 would be invalid.

I understand that part of the perfcounter code was rewritten in 1.8.6.
Prior to 1.8.6 the items for non-existent CPUs (X>4) would just return 0, even though the perf_counter instance is invalid.
With 1.8.6 and 1.8.7 I understand this has been fixed and the item should return ZBX_NOTSUPPORTED as the perf_counter is invalid. It seems to work ok, in the frontend I get ZBX_NOTSUPPORTED for those items but the agent always logs that error, always with the same 'Jýþ' string and the same 0x800007D0. The 0x0000013D is variable.