Backup SQL Server with a system backup
No, you cannot rely on system or NTFS backups as an appropriate disaster recovery strategy for SQL Server.  If all your current solution is doing is backing up the database files (i.e. mdf, ldf, ndf) and if you needed to revert to those "backups", then you could potentially run into a horrific situation where SQL Server can't use those database files.

You need to ensure that you're taking SQL Server backups (full, differential, transaction log [unless you're in simple recovery]).  This will provide you with the correct way to recover from disaster in the event you need to do a restore.

A few notes:


Store your backup files separately from the server in order to not have a single point of failure
You need to test your backed up databases for corruption(i.e. restore database, run DBCC CHECKDB against restored database).  If you have been backing up a corrupt database unknowingly for 3 weeks, and your backup retention is only 2 weeks, then you could have only a corrupt database that has been backed up