Hi, that USB modem is a Wireless Modem( like 3g modem etc.)? or a wired modem (such as cable modem or DSL)?
If is cable modem or DSL you should get the 153 kbps speed that you mention, or at least 60 to 100 kbps.
If is wireless modem, Some objects can interfere to the signal like floor, metal objects and file cabinets.
Also, if you want to know exactly which speed you have right now go to www.speedtest.net that could help you a lot.
Good Luck :)