Upgrading a viral windows 7 to windows 10

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.

I did this on a 2009 Sony Core2Duo CPU with 4GB RAM. It's faster after the upgrade to W10 but W10 is supported on this laptop. For this one, it's working great and saved the laptop from being gifted away.

Here you don't have that much RAM so iffy if it will better or worse.