If you try to update the virus defintions for ClamWin by selecting "Download Virus Database Update" and then see "Completed" immediately without new definitions being downloaded, the problem may be due to an incompatibility with the cygwin1.dll required by ClamWin and the cygwin1.dll file in use by some other application on the system, such as OpenSSH for Windows. See Incompatibility between OpenSSH for Windows and ClamWin for instructions on how to fix the problem.
You can determine which processes have the cygwin1.dll DLL loaded with the tasklist command on a Windows XP system.
C:\Program Files\ClamWin\bin>tasklist /m /fi "modules eq cygwin1.dll"
Image Name PID Modules
========================= ====== =============================================
sshd.exe 5276 ntdll.dll, kernel32.dll,
cygcrypto-0.9.7.dll, cygwin1.dll,
ADVAPI32.DLL, RPCRT4.dll, cygz.dll,
ws2_32.dll, msvcrt.dll, WS2HELP.dll,
mswsock.dll, hnetcfg.dll, GDI32.dll,
USER32.dll, wshtcpip.dll, wsock32.dll,
DNSAPI.dll, winrnr.dll, WLDAP32.dll,
Secur32.dll, mpr.dll, uxtheme.dll
switch.exe 2336 ntdll.dll, kernel32.dll, cygwin1.dll,
ADVAPI32.DLL, RPCRT4.dll, Apphelp.dll,
user32.dll, GDI32.dll
sh.exe 1192 ntdll.dll, kernel32.dll, cygwin1.dll,
ADVAPI32.DLL, RPCRT4.dll, user32.dll,
GDI32.dll
sh.exe 3836 ntdll.dll, kernel32.dll, cygwin1.dll,
ADVAPI32.DLL, RPCRT4.dll, Apphelp.dll,
VERSION.dll, user32.dll, GDI32.dll
