On a Windows XP Professional PC shared folders of network PC’s went invisible. Although the PC was connected to the network and able to access the internet through proxy.

While checking the Event Viewer i found few errors and started working on those.
The webclient service was hanging on startup and generating error with event id 7022
—————————
The WebClient service hung on starting.
—————————

I tried to start it manually from services.msc and from command (net start webclient) but it was giving following error,
—————————
Could not start the WebClient service on Local Computer.Error 1053: The service did not respond to the start or control request in a timely fashion.
—————————
The above error was resolved after installing the Dot Net Framework 3.0.

But the actual problem was there and the shared folders and shared resources (like printers) were not accessible.

There were two more errors on event viewer,
with event id 1030
—————————
Windows cannot query for the list of Group Policy objects. A message that describes the reason for this was previously logged by the policy engine.
—————————

and with event id 1058
—————————
Windows cannot access the file gpt.ini for GPO CN={31B2F340-016D-11D2-945F-00C04FB984F9},CN=Policies,CN=System,DC=mydomain,DC=com. The file must be present at the location <\\mydomain.com\sysvol\mydomain.com\Policies\{31B2F340-016D-11D2-945F-00C04FB984F9}\gpt.ini>. (Incorrect function. ). Group Policy processing aborted.
—————————

Many fixes were applied and run a full scan of Symantec antivirus but the problem was still there.

I downloaded the free version of anti-malware Malwarebyte from,

http://www.malwarebytes.org/mbam.php

Installed and updated it. Then run a full scan. It detected a Trojan and successfully deleted its registry key and file.

Registry Entry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\uqomhrgq
File:
c:\WINDOWS\system32\drivers\uqomhrgq.sys

After the reboot everything was normal and all the shared folders and resources were visible and accessible.