Automating EBS snapshot from it's instance its

Is it good idea to create periodic snapshot of the EBS volume from same instance it is attached to? Is there any downtime during snapshot process? I basically wanted to keep a regular may be daily or weekly snapshot of the ec2 instance so that If there is any virus or hacking or security issue I could spin another instance from the snapshots.

Absolutely yes. It's a good practice (personally, I consider it a must) to create point-in-time snapshots and to use them to create new volumes or restore old volumes. There is no downtime during the snapshot process. For a more detailed explanation you may take a look here, with particular emphasis on this part:

You can take a snapshot of an attached volume that is in use. However, snapshots only capture data that has been written to your Amazon EBS volume at the time the snapshot command is issued. This may exclude any data that has been cached by any applications or the operating system. If you can pause any file writes to the volume long enough to take a snapshot, your snapshot should be complete. However, if you can't pause all file writes to the volume, you should unmount the volume from within the instance, issue the snapshot command, and then remount the volume to ensure a consistent and complete snapshot. You may remount and use your volume while the snapshot status is pending.

Before doing operation involving data I think it's very important to know everything about a technology that you are going to use. So, I would like take this opportunity to put the focus on some points, taken from the official AWS EBS documentation, that are very important:

Amazon EBS volumes are designed to be highly available and reliable. At no additional charge to you, Amazon EBS volume data is replicated across multiple servers in an Availability Zone to prevent the loss of data from the failure of any single component.

If you wish to achieve greater durability, you can use the Amazon EBS Snapshot capability. Snapshots are stored in Amazon S3 and are also replicated automatically among multiple Availability Zones. You can take frequent snapshots of your volume for a convenient and cost-effective way to increase the long-term durability of your data. In the unlikely event that your Amazon EBS volume does fail, all snapshots of that volume remain intact and you can re-create your volume from the last snapshot.

Here, some notes about the durability of EBS volumes:

The durability of your volume depends both on the size of your volume and the percentage of the data that has changed since your last snapshot. As an example, volumes that operate with 20 GB or less of modified data since their most recent Amazon EBS Snapshot can expect an annual failure rate (AFR) of between 0.1% – 0.5%, where failure refers to a complete loss of the volume. This compares with commodity hard disks that typically fail with an AFR of around 4%, making EBS volumes 10 times more reliable than typical commodity disk drives.

Important details about the price:

Amazon EBS Snapshots are stored incrementally: only the blocks that have changed after your last snapshot are saved, and you are billed only for the changed blocks. If you have a device with 100 GB of data but only 5 GB has changed after your last snapshot, a subsequent snapshot consumes only 5 additional GB and you are billed only for the additional 5 GB of snapshot storage, even though both the earlier and later snapshots appear complete.

Here is why you may stay secure when you delete one of your snapshots:

When you delete a snapshot, you remove only the data not needed by any other snapshot. All active snapshots contain all the information needed to restore the volume to the instant at which that snapshot was taken. The time to restore changed data to the working volume is the same for all snapshots.

Another important advantage of snapshots:

Snapshots can be used to instantiate multiple new volumes, expand the size of a volume, or move volumes across Availability Zones. When a new volume is created, you may choose to create it based on an existing Amazon EBS snapshot. In that scenario, the new volume begins as an exact replica of the snapshot.

Ok, I think that these are some of the most important things to know when using amazon EBS. For further details take a look here. Pay particular attention on the "Amazon EBS Snapshots" section.