Imac mid 2007 low-end 20" help

I'm newbies of Imac,my Dad give me a Imac when he pass way,I'm used software 10.4.10 I can not do any thing when I search the web is asking me Administrator password,I do not have any factory software to reset is there away get get around to reset password or some one known where I can get the software,here the spe below:

Model Indentifier:Imac 7.1

Processor:Intel core2 Duo

Processor speed:2GHz

Number of process 1

number of cores 2

Memory:2GB

L2 cache(per processor) 4MB

Boot Rom Version:IM71.007A.B00

SMC version 1.20f4

thank you in advance for any help.

You really should have the install DVD to reset the password. If you no longer have it or it's lost, you can order another set from Apple customer service.

Read this Apple KB article: Mac OS X: Changing or resetting an account password

Also read this: Mac OS X - Resetting a Forgotten Account Password

Regards.