If it's exactly the machine from http://www.cnet.com/products/dell-vostro-3400-14-core-i3-350m-2-gb-ram-320-gb-hdd/specs/ and Windows 10 is offered by Microsofts GWX software, Windows 10 will work with a comparable performance to Windows 7.
If you want it to work faster (that is, faster than your current Windows 7) the 2 things to consider are: (1) more RAM (go from 2 GB to 4 GB) and (2) replace the hard disk (HDD) by a solid state disk (SSD). Those would speed up Windows 7 as much as Windows 10, so it seems it's not a priority for you, or you would have done it already.
If you now have viruses, my advice is this:
1. Make a backup copy of everything you don't want to lose (data, settings) to a USB-stick, an external hard disk or the cloud. You should have 2 different copies at least. You don't need instructions to copy files, do you?
2. Go back to factory conditions from the recovery partition or the recovery disks you made. Instructions for that are from Dell.
3. Turn off Windows Update.
4. Use a wired connection to Internet or re-enter your WiFi-key. That doesn't survive a factory restore.
5. Go to https://www.microsoft.com/en-us/software-download/windows10 and either press the button to upgrade, or (if you prefer to have installation media for later use) make a install disk or stick and boot from that to do the upgrade.
6. Install the antivirus of your choice. If the Dell came with a trial copy of some suite, uninstall that first.
7. Reinstall all application programs you installed yourself from the latest (that's better than the original) downloaded install program or from the disk you installed them from the first time.
8. Enjoy Windows 10.
1.should i install the antivirus and clean the windows and then upgrade it?
2.Does an antivirus clean a laptop?or only protects it?
3.which antivirus is better?
4.Do i need to get a back up from files, for upgrading the windows?
5.Please give me instructions for cleaning the windows 7,getting backup from files,and upgrade the laptop
6.The laptop model is Dell vostro 3400
Can the laptop run windows 10?will it be effective to upgrade it from windows 7 to 10?
thank you very much ,...and sorry for my bad english