Hi Gunjan,

You can follow the below steps to resolve this issue:

Checking the system configuration – Windows 7 uses lot of resources and space. It needs a minimum 1.5 GB DDR2 on 32-bit and 2 GB DDR2 on 64 bit versions, and it also requires a minimum of 15 GB on 32 bit and 20 GB on a 64-bit system. If Windows 7 is installed on a system with a configuration much lower than specified, it is bound to become slow and pop up regular Not Responding messages.
So, first thing, make sure that your computer hardware meets minimum system requirements.

Checking Software and Hardware – If you have installed any program or hardware after which you get the Windows 7 “Not Responding” message, remove this hardware or software. If you do not remember the name of the problematic software but remember the date when the system was working fine then you can use the System Restore tool to revert system settings to a date when the system was working fine.

Checking for Virus and Malware – Sometimes the system may be affected by a virus or malware. The system becomes slower day by day as the system resources are taken away by unwanted programs and files installed by viruses or malware. Updating the Anti-virus and running a complete system scan is the best way to deal with it.

Multiple Programs eating memory – It is also a good idea to stop all third party software and look for the error. To achieve this –

1. Click Start and type msconfig and press Enter.
2. Under the General tab, select Selective Startup.
3. Clear Load Startup Items and then select the Services tab.
4. Check the option Hide all Microsoft services and then select Disable All.

If the system starts working fine, you can enable one program at a time to determine which program is causing the problem. If this does not solve the Windows 7 “Not Responding” message, go to the next step.

Checking Indexing options – Windows 7 keeps indexing the files placed on the system for easy and quick searches, this can sometimes make the system slow and pop up the Windows 7 “Not Responding” message.
To stop the indexing follow these steps-

1. Click Start and type index and press Enter.
2. The indexing options window will display.
3. Click Modify and clear all the boxes.
4. Now click OK and close everything.

Now check for the error again, if the system is working fine you can enable one option at a time to determine which one is causing the problem.

Disable Windows Session Manager – Sometimes, the ‘Desktop Window Manager Session Manager’ might cause problems and make the system slow.
You can disable it by performing these steps –

1. Click Start, type services and press Enter.
2. Right-click ‘Desktop Window Manager Session Manager’ and disable it.

You can refer to the below article for more information on how to make your PC fast:

http://www.reviversoft.com/blog/2011/08/nine-steps-to-making-your-old-pc-run-like-new/

I hope this helps.

Regards,
Lokesh