In response to Pondus:
VirusTotal reported that the file (with the same checksum as mine) had been previously scanned, and was OK.
VirusTotal result URL:
http://www.virustotal.com/file-scan/reanalysis.html?id=0162b66ebb6802d03b759e4e1b42c178fea698eb6735bb1e8f6290aebb8baaa4-1312591320
VirusTotal last report URL:
http://www.virustotal.com/file-scan/report.html?id=0162b66ebb6802d03b759e4e1b42c178fea698eb6735bb1e8f6290aebb8baaa4-1280164504
The Malwarebytes quick scan found no problems either. (The registry issues it found can be safely ignored.) Here is the scan log:
Malwarebytes’ Anti-Malware 1.51.1.1800
www.malwarebytes.org
Database version: 7390
Windows 5.1.2600 Service Pack 3
Internet Explorer 8.0.6001.18702
8/5/2011 9:13:04 PM
mbam-log-2011-08-05 (21-12-47).txt
Scan type: Quick scan
Objects scanned: 221912
Time elapsed: 9 minute(s), 21 second(s)
Memory Processes Infected: 0
Memory Modules Infected: 0
Registry Keys Infected: 0
Registry Values Infected: 0
Registry Data Items Infected: 3
Folders Infected: 0
Files Infected: 0
Memory Processes Infected:
(No malicious items detected)
Memory Modules Infected:
(No malicious items detected)
Registry Keys Infected:
(No malicious items detected)
Registry Values Infected:
(No malicious items detected)
Registry Data Items Infected:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Security Center\AntiVirusDisableNotify (PUM.Disabled.SecurityCenter) → Bad: (1) Good: (0) → No action taken.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Security Center\FirewallDisableNotify (PUM.Disabled.SecurityCenter) → Bad: (1) Good: (0) → No action taken.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Security Center\UpdatesDisableNotify (PUM.Disabled.SecurityCenter) → Bad: (1) Good: (0) → No action taken.
Folders Infected:
(No malicious items detected)
Files Infected:
(No malicious items detected)
In response to polonus:
Yes, I knew PWMIDTSK was related to Power Management, and did not intend to get rid of it. The VirusTotal scan establishes that it is what it appears to be. There are in fact no instances of PWMIDTSK.EXE spawned, according to Task Manager. There are several instances of svchost.exe running, which is normal for Windows XP, and Avast has reported that one of them is trying to open PWMIDTSK.EXE, which Avast has suddenly decided is potentially unsafe.
In response to all:
So my conclusion is that PWMIDTSK.EXE is not unsafe. There is no evidence that it has become infected with any malware. The Avast popup appears to be a false positive. It would be nice to understand why Avast suddenly decided that this file might be potentially unsafe. Could an update to the Avast virus database result in a false positive of this sort? Who knows. But I will proceed by telling Avast to open the file normally.
Thanks to all for their assistance.