Several readers report that a 4.5.1 version is on the way. Jim Carr notes that it should at least fix the following matters, as described in an email from Symantec Tech Support:
"The February virus definitions files do require a little more space in memory which could generate an error if things are not set properly. If it is 'segment loader' error, then try trashing your SAM Virus Defs and reinstalling them, especially if you overwrote the old ones when updating them rather than trashing the old ones before installing the new ones. Check your cache setting in the Memory control panel. If it is not at 32K per Mb (example 256K if you have 8 Megabytes of RAM), if it is at say 96K then increase it to the suggested setting (to a maximum of say 1024K or maybe 2048 if you had say 64Mb of RAM or more). Also check your hard disk with Disk First Aid and Disk Doctor if you own it. Disabling the Modern Memory Manager can also work or loading the SAM Intercept later in the load order." The email concludes with a description how to change SAM's SYSZ resource in ResEdit - as a means of resolving some of these issues. Frankly, I would rather just wait for 4.5.1.