Windows Backup failed as another backup or recovery is in progress

i have set up backup schedule on our server. SQL Server 2008 to 01:00am on windows server 2008 R2 to 4:00am. Sql backup runs well, but system backup ends sometimes with error.

The error is : http://technet.microsoft.com/en-us/library/dd364768%28WS.10%29.aspx

I mean, that is caused by SQL Server, because it unexpectedly runs backup.

this is from sql server log (it runs for every database):

Date 4.2.2010 4:00:21 Log SQL Server
(Current – 29.1.2010 23:25:00)

Source spid72

Message I/O is frozen on database
master. No user action is required.
However, if I/O is not resumed
promptly, you could cancel the backup.

and

Date 4.2.2010 4:00:24 Log SQL Server
(Current – 29.1.2010 23:25:00)

Source Backup

Message Database backed up. Database:
master, creation date(time):
2010/01/29(23:25:32), pages dumped:
370, first LSN: 859:56:37, last LSN:
859:80:1, number of dump devices: 1,
device information: (FILE=1,
TYPE=VIRTUAL_DEVICE:
{'{17B91D5C-9968-4D11-A7F1-1A31523D32F0}25'}).
This is an informational message only.
No user action is required.

My questions is:

Why runs sql backup with windows backup?
How can i dissolve this that errors?

Thank you a lot.