...."pctspk.exe is a diagnostic program for PCTel modems. It doesn't seem to be recquired. Some people have said their modem still works after closing this program while others have reported problems.? "
Your copy of pctspk is possibly corrupt and Windows is replacing it's working copy. Alternatively the real pctspk has been replaced by a trojan. Or it's on a dodgy bit of the disk and Windows sometimes reads it correctly and sometimes not.
"Description: File pctspk.exe is located in the folder C:\Windows\System32. Known file sizes on Windows XP are 86016 bytes (55% of all occurrence), 163840 bytes, 167936 bytes, 180224 bytes, 74752 bytes, 173056 bytes, 155648 bytes, 68608 bytes, 176128 bytes.It is a Windows system file. The program has no visible window. It is a Microsoft signed file."
I suggest that you find it and RENAME it to pctspk.old (rename so that you block a potentially bad sector). Windows SHOULD replace it with a clean version from the dllcache folder.
You may need to disable WFP in order to get a good copy in - see
http://www.winguides.com/registry/display.php/790/
and
http://www.updatexp.com/windows-file-protection.html
Getting a good copy - there should be one in one of the .cabs in the XP install disk - or try Google.
Edited to add:
On my XP Pro SP2 there is a copy of pctspk.exe in C:\WINDOWS\Driver Cache\i386\driver.cab - size 86016, date 17/08/2001 10:36 PM.
Winzip can open .cab files and extract it. If you have problems finding it, pm me and I'll send you a copy.