Hello Julin
Cisco does not update or support software/hardware which has reached End of Life/ End of support.
3550 was declared EOL in 2005, and The last date that Cisco Engineering may release any final software maintenance releases or bug fixes, was may 2 2007. . After this date, Cisco Engineering will no longer develop, repair, maintain, or test the product software. (taken from CCO)
Refer to the EOL document of 3550.
http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps646/prod_end-of-life_notice0900aecd8029f777.html
Hope this helps.. all the best.. rate replies if found useful.
Raj