Windows 7 taking 5-10 minutes to boot up.

Hello toashishonly and welcome to Seven Forums.

Sometimes boot problems are related to malware (virus, spyware, etc.) If you haven't done so, run a full scna with your anti-virus of choice. Even if that scan comes back clean (no malware detected) I'd follow up with some additional free on-demand scanners like Malwarebytes, SuperAntispyware, ESET Online Scanner, etc.

You could also see if this Startup Repair tutorial helps:

Startup Repair

As to CPU usage, it's not uncommon for System Idle Process to be at 98% if the computer is turned on and you're not doing anything else with the computer. When you start another process the new process will usually use a smaller fraction and System Idle Process may drop down to 0%. This article goes into more detail.

The best protection to not have to reformat and reinstall everything is to make regular backups of all your important files and folders. I prefer doing a full system image which also backs up the entire operating system, all updates, etc. A lot of people recommend the free Macrium Reflect.

I have scanned with kaspersky lab antivirus and malwarebytes but it detects nothing and cpu usage don't exceeds even when I open various softs. when I open various softwarea at same time then my computer stucks but cpu usage remains fixed at about 10-12% .

Windows 7 has a built in utility called Resource Monitor. It gives real time performance and can help isolate problem areas. I'd look at the CPU and Memory tabs. You may be maxing out your RAM (many people recommend 4GB.)

Resource Monitor

Well, things look normal to me. Couple more things you can try just for general cleanup and performance.

Disk Cleanup - Open and Use

Disk Defragmenter - Open and Use

If still having issues, one of the Forum experts prepared this general troubleshooting guide. It has a lot of good suggestions and it's not something that a person can do in just a few minutes (or even a few hours.)

Troubleshooting Steps for Windows 7