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Thanks for teaching me this method.
my wallet is now rescanning.
Good, let us know when its done.
When i check the address that i sent the 2.5btc to, looks like someone spent it already so i m sure that is someone else's wallet. but why my wallet sent my btc to this address instead of the correct address?
yes, i have cleaned my wallet by replacing with an early backup that is long time ago(~2 years), but i found out the wallet missing most of my addresses so i change it back to my original wallet file with conflicted transaction. Apart from that, no one else have touched the wallet file.
Old backups might not have all private keys, you should consider doing regular backups. I do them ~once a month even when I did not use a new address. Just in case.
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It is possible that you have a virus on your computer. There have been several cases of various viruses that will steal your Bitcoin wallet thus allowing the attacker to spend your funds. This may have happened, so get an antivirus and scan your computer to make sure that there are no viruses.
It certainly is possible. Installing anti virus software on an infected system is usually no solution though. You would need a clean boot CD/DVD to scan from.